Obturation question

[Black Jaque Janaviac]

Hello. First post to the CB forum. I like what I see here.

Would anyone here be able to explain how Section Density affects obturation?

[45nut]

i am not the guy to answer the question, but I will add my welcome to CB.

[Bass Ackward]

Hello. First post to the CB forum. I like what I see here.

Would anyone here be able to explain how Section Density affects obturation?


BJJ,

One word: Inertia.

The heavier the bullet per caliber, the higher the SD. The higher the SD the more weight. The heavier an object, the longer it tends to remain at rest. So it will throw more pressure. More pressure, the more obturating force.

[Larry Gibson]

BJJ,

One word: Inertia.

The heavier the bullet per caliber, the higher the SD. The higher the SD the more weight. The heavier an object, the longer it tends to remain at rest. So it will throw more pressure. More pressure, the more obturating force.

Bass

I'm impressed.......not much left for me to say.

Larry Gibson

[HORNET]

Dang, Bass.
Short, simple, and to the point.....you feeling OK??:kidding:
Welcome to the internet home for the lead addicted, Black Jaque. Lots of nifty info available on here.:brokenima

[Bret4207]

Bass- Break a couple of fingers or something? Short and sweet! And rather well put too. Wish I could explain stuff like that.

Welcome to the asylum BJJ!

[Black Jaque Janaviac]

OK Bass. Now how about if the pressure variable is controled?

Say both loads are generating the same peak pressure. Will both types of bullets exhibit the same amount of obturation?

[35remington]

What "both" are you referring to? No examples were given.

[Bass Ackward]

OK Bass. Now how about if the pressure variable is controled?

Say both loads are generating the same peak pressure. Will both types of bullets exhibit the same amount of obturation?


BJJ,

Trick question was it. Now you want me to condense the secrets of shooting and why this board was formed into a single post. :grin: And you know I am a man of few words too. :grin:

Control is exactly what we, as reloaders, HAVE and strive to do.

Imagine you look up in your rear view mirror and see you are going to be rear ended. The other guy is going 25 miles per hour. Would you rather be stopped with your brake applied or .... be accelerating when he hit you? Less damage that way right? Same thing with a pressure wave on a bullet. Cast is simply more vulnerable (weaker) and more critical to control than jacketed.

Is Bullseye a good general use powder for a 7MM Mag? Why? Because peak pressure from Bullseye (parked and brake applied) in a 7 Mag would produce far less velocity than if you used say .... 4831 (accelerating) to the same exact pressure peak. That's why you won't find Bullseye listed as a powder choice for any bullet weight for 7 Mag in any reloading manual.

So the logic of bullet weight works the same way with cast as with jacketed. The heavier you go with jacketed, the slower the powder you must use to get peak power and velocity. Cast is magnified even more because it is weaker. (more sensitive to obturation.)

Make sense?

And for the rest of you guys, shame on you. Insinuating I am long winded. :grin:

[454PB]

That's more like it!

[fourarmed]

Think of it this way: The base of the bullet starts moving before the tip of it does. The longer the bullet, the farther the base will move before the tip starts to move. Since lead is incompressible, the more the bullet foreshortens, the more its diameter expands. Most of that foreshortening is at the base, so a longer bullet will obturate more. If the obturation was uniform the entire length of the bullet, length would not matter.

[Black Jaque Janaviac]

What "both" are you referring to? No examples were given.

Both bullets of high S.D. and bullets of lower S.D. Sorry for the poor wording.

[Black Jaque Janaviac]

Bass,


Is Bullseye a good general use powder for a 7MM Mag? Why? Because peak pressure from Bullseye (parked and brake applied) in a 7 Mag would produce far less velocity than if you used say .... 4831 (accelerating) to the same exact pressure peak. That's why you won't find Bullseye listed as a powder choice for any bullet weight for 7 Mag in any reloading manual.


Hmm. After mulling this over a bit, perhaps obturation is affected by the duration of the pressure above the plastic limit of the lead. (pant-pant-pant, that's tough on the old brain to toss big words like that in the same sentence)

See, what I've been noticing is the exact opposite of what you guys are describing. In my .357 the 200 grain boolit obturates less than the 158 grain boolit. The 200 grain boolit is over a 13 grain charge of Lil' Gun, whereas the 158 grain boolit is over a 17 grain charge of Lil'Gun. Yet, the primer flattening is greater with the 200 grain boolit.

Here's my stinking - er- thinking. The heavy boolit generates a higher peak pressure overall but is a shorter curve. The lighter boolit generates lower peak pressure but has a longer curve (more powder to burn). Both loads exceed the plastic limit of lead, but the lighter weight boolit is subjected to that excessive pressure for a longer period of time before the pressure drops below the plastic limit.

The reason the primers don't flatten in the 158 grain boolit load is because the peak pressure does not exceed the plastic limit of the brass or nickel of the primer (must be higher than lead).

This of course assumes that the amount of time the lead is exposed to pressure affects the amount of obturation.

Does my hypothesis sound reasonable? Or do I need to do more reading?

[Bass Ackward]

Here's my stinking - er- thinking. The heavy boolit generates a higher peak pressure overall but is a shorter curve. The lighter boolit generates lower peak pressure but has a longer curve (more powder to burn). Both loads exceed the plastic limit of lead, but the lighter weight boolit is subjected to that excessive pressure for a longer period of time before the pressure drops below the plastic limit.

The reason the primers don't flatten in the 158 grain boolit load is because the peak pressure does not exceed the plastic limit of the brass or nickel of the primer (must be higher than lead).

This of course assumes that the amount of time the lead is exposed to pressure affects the amount of obturation.

Does my hypothesis sound reasonable? Or do I need to do more reading?



BJJ,

Ever play baseball? Here comes a curve. you make the assumption that weight is the only factor here.

How does bullet design affect obturation and thus pressure limits? Are all 158 grain or 200 grain bullets for that matter, the same?

Your turn. :grin:

[twoworms]

BJJ,

Welcome to CastBollits, you're like it here. If you have any questions about casting, ask away. Lots a good info here and very little bull along with it.

Tim

[leftiye]

BJJ, My guess is that the 200 grain load both produces a higher pressure, and a longer duration as the boolit moves out of the way of the powder charge more slowly. What you are describing is simply higher pressures. The 200 grainer is a bit much for the standard .357 case, and therefore problematic to get going smartly. Just as the three hunnert grain boolit overpowers the .45 long Colt cartridge, being too much lead to move very fast within pressure limitations.

So what obturation are you talking about? Primer flattening, or boolits expanding when kicked in the butt by a powder charge? Longer boolits have a longer (by definition) column of lead on top of the boolit base. This causes the internal pressures upon the lead to be greater as the boolit is accelerated. As B.A. said, the inertia of the boolit front parts bears against the base of the boolit. If pressure is greater than the elastic strength of the alloy, the boolit slumps starting at the base and working forward. In extreme cases the whole boolit slumps to the rear. Someone here (I think it may have been 44man) posted some pictures of SWCs that had been cast of soft lead and fired with maximum pressure load. They had been re-formed into truncated nose boolits by the aforementioned process.

[cbrick]

Bass Ackward's first post was pretty much on.

To expand on it some, Ken Mollahan in Handloader #73, May June 1978 did an excellent article, "Limitations of Cast Bullets. After a couple of years of testing rifle rounds he came up the theory of a lower SD making it easier, even possible to acheive jacketed velocity and accuracy with cast in rifles. As the SD was increased (by bore diameter or bullet length) the velocity had to be decreased or accuracy was lost. The small bores, 22's, 6mm's etc where fairly easy, as the SD and or bore diameter and bullet weight increased it became more and more difficult to maintain accuracy without decreasing velocity. One of his theories was the larger area in square inches of the bullet base of larger bores that recieve the pressure plus the bullet length, or sectional density (SD).

I think I explained that fairly accurately without digging out the article and re-reading it. This article is the first I have ever found on the theory of keeping the SD low while increasing the velocity and it seems to have merit.

My own SWAG is that as bullet weight is increased (as pointed out already) the base tries to move before the nose, the nose can slump or be deformed even with designs of short fat noses and ogives, especially if this should occur in a fairly loose throat before the relative safety and confines of the bore. Only so much can be done with alloy hardness (strength) before that dreaded blow by occurs. This is why the slower powders usually work better when striving for higher velocity, get a more gentle continuous push until maximum velocity is acheived or get smashed in the heiny with a sledge hammer.

Rick

[BAGTIC]

If you used a bullet that was large enough to start with you wouldn't need to worry about bumping up the bullet's diameter.

[DLCTEX]

If anyone doubts the elastisity of lead, I offer this experience. I told in another thread of my son Dave firing my 45 acp and 22 Ruger pistols underwater. The 45's were full metal jacket and only had rifling marks to show it had been fired, but the 22 bullets were stretched to almost twice their normal length, obviously by the drag created by the flat bases. These were Wally World $9.00 a brick stuff, probably Federals as they were plain uncoated lead. Point being, if drag can stretch lead, how much more can pressure obturate it? I have no problem believing even relatively hard lead will obturate. Just my dos centavos. Dale:Fire:

[BlackRifleShooter]

If anyone doubts the elastisity of lead, I offer this experience. I told in another thread of my son Dave firing my 45 acp and 22 Ruger pistols underwater. The 45's were full metal jacket and only had rifling marks to show it had been fired, but the 22 bullets were stretched to almost twice their normal length, obviously by the drag created by the flat bases. These were Wally World $9.00 a brick stuff, probably Federals as they were plain uncoated lead. Point being, if drag can stretch lead, how much more can pressure obturate it? I have no problem believing even relatively hard lead will obturate. Just my dos centavos. Dale:Fire:


Ok first off Dad, I think your recollection of the type of .45 pistol used in the underwater firing. That point is mute though. I wont use any fancy words like obturate, but I will say that the 22 LR lead bullets did "stretch". The base of the bullet and the very tip were the least deformed. We recovered a dozen or so bullets from the pool and to the naked eye they were all identical in shape. The bullets were elongated 1/2 to 1/4 their original size and the tip of the bullet was curved up. Kind of like a minature lead bananna.

[jim4065]

For what it's worth, I don't think that elasticity is what we're looking at here. If memory serves, elasticity refers to the ability of a substance to return to its former shape after being distorted. The lead has almost no elasticity - that's why it stayed in the curved shape. As a side note - steel is more elastic than rubber.

That's about all I retained from Physics 101. :roll:

[DLCTEX]

Black Rifle Shooter, I bow to a younger memory, do you mean it was your Glock in 45 you fired? I remember the 22 bullets being really long, and smaller in the middle, correct? Maybe the term we need is ductile, rather than elastic, or maybe plasticity rather than elasticity?Malable? What we mean is it squashes out bigger when we shoot it.???? Dale .. or Dad in this case.

[BlackRifleShooter]

Yes it was my Glock G21 .45 acp and yes the 22 bullets were skinnier in the middle it looked like that was all that stretched was the middle section, not the base or the nose.

[Black Jaque Janaviac]

How does bullet design affect obturation and thus pressure limits? Are all 158 grain or 200 grain bullets for that matter, the same?


No. Bullet designs are not the same. The design can greatly affect bore friction which will have an impact on obturation - I suspect this has greater affect than just the weight/inertia. More friction means greater resistance to forward motion, which in turn builds pressure.

Also, the diameter in the lube grooves or depth of the grooves, however you want to put it, will affect obturation. A deep groove should exhibit more obutration than a shallow groove, as a percentage of it's original dimensions.

But in either case, the 200 grain bullet should have greater obturation, since it's longer w/ more bearing surface and has deeper grooves. But the 200 grain bullets exhibit LESS obturation.


OK. So obturation is the movement of mass perpendicular to the direction of the forces applied. When a bullet is fired a force pushes at the rear, and a force (inertia/friction) pushes back from the front. The lead responds by moving to the sides.

Higher S.D. means more mass. More mass means more inertia. But that's not just inertia in a line down the bore, the rules of inertia would also apply to the bullet's resistance to obturation too. Wouldn't it? I mean all obturation is, is the movement of mass.

[Larry Gibson]

BJJ

Let me get this right; the heavier bullet has less obturation than the lighter bullet yet the 200 gr bulllet load exhibits more pressure - right? That makes perfect sense as the one thing not mentioned in all your posts or some of the answers is the velocity of the two different loads. From the load data you provide the lighter bullet is obviously much faster than the heavier. The faster a bullet accellerates (velocity) the greater the inertia acting against that accelleration, hence the greater the obturation. It is as simple as that with all the other things Bass and others have mentioned.

Larry Gibson

[leftiye]

So, We all kinda missed the operant force here by a smidgin. It is the rate of acceleration that produces the "G" forces that result in the front part of a boolit resisting the push from the rear (inertia) NOT Pressure. In other words how fast the rear of the boolit is accelerated determines how great the rearward force of the inertia of the front part of the boolit becomes. Pressure results in acceleration, but with lower sectional denstities rate of acceleration can be so great as to produce higher obturation even at lower pressures.

[njmj]

Dale, Dad, or black rifle shooter,
When you fired the 22 lr bullets into the pool, was the pistol out of the water or submerged? I'm sorry I have to ask this but the appearance of the bullets have me really curious. Thanks, njmj

[DLCTEX]

The guns were fired underwater. Dave had watched film of Navy seals firing under water and decided if they could do it, so could he. The bullets don't travel far, it was a 15 ft. pool and the bullets didn't reach the far side where he had placed a heavy timber. The concussion will kill fish at close range, or if they are in a hole or pipe. You don't want trapped air in the barrel, just water. No indication of pressure problems as I remember, maybe Dave will chime in here, as most of this was conducted without my knowledge until after the fact. Dale:Fire:

[JSH]

obturation, this kind of discussion gives me a headache,lol.
Is this fact, fiction or theory?
If the slug starts out at .XXXX larger, what is to slug up if is already that much over bore to speak?
How do we know the base really slugged up and it just didn't swell the base s bit from impact?
I am not near the experiance of most of you here, I always try to look at these threads with an open mind and ear. I have shot some CB's that were sized as normally thought to be correct. I have also shot quite a bunch that "shouldn't" shoot very well that did exceptionally well.
I do think what happens to the base of ANY projectile may never really be known? That is until we can shrink ones self to stand the pressure and be inside the case to observe hands on.

I am talking obturation across the board, not just one caliber and one type of gun.
IMHO, this would take place better in a revolver than any other type of gun? My thoughts being it slams into the forcing cone with the energy behind it pushing the base. But, then on the other hand, reading fourarms answers and some of the others this would not be correct either. Then throw the sizes stated on the FA chamber, forcingcone and bore..........................................
Maybe like a bumble bee? He really should not be able to fly.

Now I really have a headache and need another cup of coffee,lol. I am going back to the K.I.S.S theory, and if it ain't broke don't fix it, nor ask how it does that.
Jeff

[felix]

Best not to forget F = M * A in any discussion. The first post by BA and the last by Lefiye should be combined into one post. ... felix

[njmj]

Dale,
That's what I kinda thought. I believe the reason the 22 bullets got to be the shape they are is because as the bullet accelerated down the 22 lr barrel full of water, the water was squeezed between the bullet and sides of the barrel. Water being incompressible and lead relatively soft, the water actually was run over by the lead bullet and forced to squeeze up the middle. In an effort to prove this, if we fired the gun under water but say inside a plastic bag so there was no water in the barrel to begin with, the shape of the bullet would be normal. Well, maybe not normal but at least what we are used to seeing. Real interesting stuff. Thanks, Marshall

[njmj]

I did a test trying to figure out if the hardness of my bullets matched the pressure level for proper obturation, whatever proper is. I was using a BPCR in 40-65. 60 grs. of Goex 2F. I cast some 38-55 bullets (.379 dia.) to shoot out of .409 groove dia. bore. I cast 5 bullets each out of pure lead, wheel weights, and linotype. I used a 14 foot long wood crate full of sawdust with the close end knocked out. The bullets were fired into the crate from about five feet away. After putting out the smoldering sawdust the bullets were recovered. The pure lead bullets were all .409 dia. and short. The wheel weight bullets varied between .389 and .400, and the linotype bullets measured around .379. The interesting thing about the linotype bullets was that the lube grooves looked like saw teeth with the teeth pointing forward. At first I thought this was from the bullet entering the sawdust. but then realized that the teeth should be going the other way. What made the lube grooves (actually the driving bands)look this way was the gas rushing past the bullet on its' way out the barrel.
The conclusion from this is that with an alloy like wheel weights there is enough pressure to obturate the bullet to seal the bore. njmj

[Black Jaque Janaviac]

It is the rate of acceleration that produces the "G" forces that result in the front part of a boolit resisting the push from the rear (inertia) NOT Pressure.

***sigh***

I'm not sure I'm understanding this any better.

The pressure IS the "push from the rear". Multiply pressure (PSI) by the sq. inches of surface area of the base of the bullet and you get the force from the rear.

For every action there is a re-action or however it goes. So if the pressure is the same in both loads the G-forces should be the same too? Or what am I getting wrong?

[leftiye]

Yes, the pressure IS the push from the rear. THAT is only felt by the base of the boolit. What is felt by the rest of the boolit is acceleration. The higher the RATE (greater increase in velocity, and/or over less time) of acceleration, the greater the force is pushing you back into the seat, or in this case the harder the front of the boolit pushes back against the rear of the boolit.

In some cases, at the same pressure, a lighter boolit will accelerate at such a greater rate compared to a heavier boolit that even with less sectional density, the fail strength of the metal will be reached, and obturation will take place. Unusually, in this case the supposed same pressure would not accelerate the heavier boolit fast enough to cause it to obturate. This is an unusual case, I feel, just as it seemed to you when you started this thread.

The pressure is the same, but the force of inertia is greater (per unit mass) when the rate of acceleration is greater.

[Bass Ackward]

Bass,

See, what I've been noticing is the exact opposite of what you guys are describing. In my .357 the 200 grain boolit obturates less than the 158 grain boolit. The 200 grain boolit is over a 13 grain charge of Lil' Gun, whereas the 158 grain boolit is over a 17 grain charge of Lil'Gun. Yet, the primer flattening is greater with the 200 grain boolit.

Here's my stinking - er- thinking. The heavy boolit generates a higher peak pressure overall but is a shorter curve. The lighter boolit generates lower peak pressure but has a longer curve (more powder to burn). Both loads exceed the plastic limit of lead, but the lighter weight boolit is subjected to that excessive pressure for a longer period of time before the pressure drops below the plastic limit.

The reason the primers don't flatten in the 158 grain boolit load is because the peak pressure does not exceed the plastic limit of the brass or nickel of the primer (must be higher than lead).

BJJ,

I have been thinking how to answer your question since you obviously feel that you have a contradiction with these two bullet weights. One more thing that you might be experiencing can come from the powder and ignition. In short, you are getting a different burn rate or burning characteristics by using the same powder for both weight slugs.

Lil'Gun is in a strange range with powders. By that I mean that it is generally accepted that as you go to a slower powder in any cartridge, you can either increase the charge or you simply get less pressure. If you ever notice, you get less pressure with H-110 /296 and charge weights are higher than if you drop to 4227 or Lil Gun. Most powder companies try to get powders to burn linear within certain pressure ranges, but they don't always do that at lower pressures. LilGun was designed to be a top end powder as are most balls. Thus you hear they burn cooler which is another way of saying they are hard to light OR .... simply that they don't burn well or consistently without the prescribed pressure.

Olin had to pull Win 630 because it was erratic under different scenarios. Well obviously, you are going to have more time to get good ignition and pressure up with the 200 grain bullet. While the 158 might be kind of light for that speed under the particular situation you loaded. Especially if you are shooting in a revolver and trying to slug up. You may be making the gun essentially a blow gun with the 158 grain. And essentially, you aren't producing the pressure with the 158 grain that you think you are, or at least the reloading manuals tell you.

I use a computer program called Quickload. One of the reasons I like it so much is that it predicts "perfect" world conditions. In other words, if everything is as it should be, your velocity should be " X " amount. If I am below that figure more than say 50 fps, then I am probably causing some error. Maybe my bullet is too hard and I am not slugging up fast enough to seal, maybe my case necks were dirty and I didn't get good neck tension, maybe my brass is losing anneal, or I am using too slow of a powder. But at least it tells me I have a problem and I need to investigate. By the same token, if I am higher than 50 fps, then my slug may be too soft, my lube right on the edge, my bullet diameter too big, or a host of other things. Bottom line, I need to look around.

Unfortunately, LilGun is not listed in Quickloads data base yet or I would run the projection and give you figures. But with the 158 grain bullet and 4227 which I believe is easier to ignite, your powder burn rate is only 65% out of a 4" and 72% out of a 7 1/2 under perfect world conditions. Just as a point of reference I save all my loads. When I finally develop a good load, I mysterously find that my burn rate is between 85% and 90% for PB. Higher than that and you are going too fast in powder choice making it hard on the bullet base, and slower than that and you are going to get wilder ES. So I would say a faster powder is in order for the 158.

So bottom line, I think you are seeing a powder related phenom and confusing it. This is essentially what Leftiye was telling you about it being simply higher pressure. I just had to use a few more words to get there. :grin:

HTH's.

[Black Jaque Janaviac]

Leftiye,


The higher the RATE (greater increase in velocity, and/or over less time) of acceleration, the greater the force is pushing you back into the seat, or in this case the harder the front of the boolit pushes back against the rear of the boolit.

This is what I'm not getting. F = M x A. So when Acceleration (A) is increased, the G-force (F) would also increase; exactly as you describe. ACCEPT that in this scenario we are ALSO reducing the mass of the bullet (M). That would serve to mitigate the Acceleration and thus keep the G-force the same. This would be born out by the assumption that the pressure is the same.

I still think that the duration of the exposure to that part of the pressure curve that exceeds the plastic limit of lead has something to do with it.


Bass,

And essentially, you aren't producing the pressure with the 158 grain that you think you are, or at least the reloading manuals tell you.

OK, but this is the confusing part, the 158 grain bullets are exhibiting more obturation than the 200 grain bullets. Where the lube grooves in the 200 grain bullet are nearly unchanged, the lube groove in the 158 is all but non-existant.

I'll manufacture some numbers just to illustrate my point. Say the plastic limit of my alloy is 18,000 PSI. Now say the pressure peak of my 200 grain bullet load is 25,0000 PSI, but the pressure curve is more of a sharp spike, rather than a broad curve. This spike exceeds 18,000 PSI on it's way up to 25,000 PSI, but then quickly drops below 18,000 PSI in a matter of 0.0001 second. This would be enough to obturate the lead some, and flatten the primer.

Now the 158 grain load peaks at only 20,000 PSI, but forms a nice broad curve because there's more powder. The duration of the pressure that is above the plastic limit of the lead (18,000) is say 0.001 second, a full ten times greater than the duration of the 200 grain load. If the plastic limit of the primer is something like 22,000 PSI, the load never exceeds that so the primers never flatten.

I would imagine that it takes time for the lead to flow and for the lube groove to expand out. If the pressure drops before the lead completely fills out the lube groove, then the lead will cease flowing.

Perhaps I should post some photos.

[felix]

BJJ, no need to post photos, because you are correct by referencing the F=M*A equation, however sorta' incognito. Time is a function of the accelleration parameter. Always keep in mind that these motion "formulas" are dynamic in that the values are always changing microscopically in terms of time when any one value changes from zero. ... felix

[Bass Ackward]

Bass,


OK, but this is the confusing part, the 158 grain bullets are exhibiting more obturation than the 200 grain bullets. Where the lube grooves in the 200 grain bullet are nearly unchanged, the lube groove in the 158 is all but non-existant.


BJJ,

OK. Now we get back to bullet design. Elmer Keith used to harp on square grooves because they would not compress under pressure. His claim was that round bottom grooves collapsed with less pressure. What do yours look like on both bullet designs? How deep are the grease grooves?

Then we have the width of the base band that determines strength. A wide band has more strength because it simply is wider. But if you have a good seal and aren't trying to bump up bore sized bullets, then a narrow band filled with incompressible lube can be stronger. So .... this really depends on shooting technique which is dictated by the gun and bullet design you are shooting.

And we also have bearing length. I like a lot of bearing length when possible because it will be supported by steel. The support of this steel will maintain a certain amount of friction, thus the longer bearing length .... helps keep more weight off the base compared to the same weight bullet in an unsupported design like a semi wadcutter.

Without going into the debate about ballistic coefficients, we can have stronger or weaker designs that are going to require a different hardness at higher pressures even in the same weight bullets.

Could some of this be why? Were both sized the same and shot using the same technique? There's more to consider.

[leftiye]

BJJ, What I said is simply a (one) posible way that the results you got could be made to happen. What B.A. has said makes up several other ways that what you've experienced could be brought about. YOU may have to do more experimenting, or look at these possibilities (and maybe some others) more closely before you come to conclusions, or possible answers as to what is causing the lighter boolit to obturate when the heavier one doesn't. That's what is called fun (as in all the fun one can stand).

[Black Jaque Janaviac]

Bass, Leftiye,

Thanks guys.

The 200 grain bullet is from a Lyman 358430, their 195 grain roundnose. It has three, narrow, roundbottom grooves. It does obutrate some, just not as much.

The 158 grain bullet is from a Lee 358-158-RF. It has one, flat bottomed groove which is considerably wider than the Lyman bullet. The Lee is a typical "cowboy action" style roundnose flat point.

Both bullets are .360 as cast and I don't bother sizing 'em.

[Bass Ackward]

Bass, Leftiye,

Thanks guys.

The 200 grain bullet is from a Lyman 358430, their 195 grain roundnose. It has three, narrow, roundbottom grooves. It does obutrate some, just not as much.

The 158 grain bullet is from a Lee 358-158-RF. It has one, flat bottomed groove which is considerably wider than the Lyman bullet. The Lee is a typical "cowboy action" style roundnose flat point.

Both bullets are .360 as cast and I don't bother sizing 'em.


BJJ,

I read and re-read this all again. Clearly, you have created a set of conditions where weird stuff happens. I can't explain the flaw. Just understand one is there.

What everyone has told you follows what I have read and experienced. Spank something harder and it deforms more. Then again, I have seen stuff that others claim I must be drinking on. So go from there. But whenever I run into stuff like that, I change something until rationality is restored.

If you don't mind, what kind of lube are you using and what's the mix?

I went out and dug up some of my 358RFs to see what I get because I run a soft 20-1. The grease grooves measure the same width as cast. But 28,000 is as high as I have gone because of the sound. I use the 357 sort of like a pregnant, wind resistant 22. I get a fairly descent trajectory and more humane knockdown than a 22 without having to crank on it. And I can shoot it on those days where arthir has my attention.

[Black Jaque Janaviac]

Clearly, you have created a set of conditions where weird stuff happens. I can't explain the flaw. Just understand one is there.

OK. I don't feel so stupid now. I can accept anomalies and mysteries. I feel stupid when everyone seems to have all these elementary explanations that make perfect sense to them but doesn't match up at all to me.

I don't think I will take all the blame for creating a "set of conditions where weird stuff happens" though. I'll blame that on Hodgdon thank-you-very-much. I don't know of ANY powders that can generate velocities equivalent to H110 with only 25,000 CUP!! I find that rather wierd and mysterious. So it's no big surprise that Lil' Gun produces some other anomalies.

Attached is a photo of the Lee 358-158-RF. The one on the left was loaded over 17 grains of Lil' Gun. The one on the right was loaded over 13 grains of Lil'Gun and is not part of this discussion. The one in the middle is not fired.

I'll post a photo of the Lyman 358430 later.

[Black Jaque Janaviac]

My lube mix is 50% rendered and rinsed deer tallow, and 50% beeswax for both bullets. The deer tallow is already a pretty hard fat so I get by with 50%.

[leftiye]

BJJ, Your boolit on the left seems purty well transformed compared th the virheen boolit in the middle. How did you recover the boolits?

At first I glance was inclined to suppose that it had muy good pressure (which it may have had), then I wondered if the deer tallow lube had contributed to scuffing or something. This is a lube usually reserved for Black Powder shooting. It's obvious that your lead is pretty soft as the nose section has slumped some and this only happens at higher pressures, and with softer alloys. What is the alloy?

Best accuracy will probably come with booits transformed (deformed) as little as possible. That means low (relatively speaking, and relative to the alloy) pressures, and enough hardness to prevent deformation. Actually, unless you want expansion on meat, you can go as hard as you want if sized correctly. Size to fit the chamber throats, and maybe .001" larger. You may already know this, I'm not assuming you don't, just running through the procedure. Wheel weights are about 14 BHN, and up to 22BHN if heat treated or water quenched, so expensive alloy is not necessary.

At 25,000 psi you have a very reasonable pressure for moderate loads in that caliber. I'd be a little nervous about whether you're getting uniform pressures (and/or uniform ignition) with that powder at that pressure though. It likes 40,000- 44,000 psi. much better, and shouldn't be loaded down too much, or SEEs might happen (no fun!). Have you tried 2400?

Maybe this can afford you some help, or if not maybe something more to chew on. Best of luck.

[BAGTIC]

Heavier bullets do NOT produce higher pressures. Maximum pressure is determined by the pressure limits for the cartridge, regardless of powder. Red Dot and H110 can be loaded to the same pressures. Red Dot just reaches it with a lighter charge. WE, not the powder determine the pressure.

How many people use the same powders for lightweight and heavyweight bullets, for example .357 Magnum 110 grain/220 grain?

The faster burning powders will produce a faster rise in pressure and that may be desirable for bumping up a bullet. By the time slow powders reach maximum pressure the bullet is already moving pretty good so the effects of inertia are already largely overcome.

[Bass Ackward]

I feel stupid when everyone seems to have all these elementary explanations that make perfect sense to them but doesn't match up at all to me.


BJJ,

You obviously are a smart fella. And I assume you figured most of this before you posted. This was a test wasn't it? :grin:

It's the accumulation of statistics that makes this sport safe. Enough people have safely pulled the trigger enough times on a potentially explosive charge to say that you will be OK if you do it the same way too. Because we have no laws in this sport, that foundation of .... elementary explanations as you call it, let's us understand when something is not .... normal. Then we have to STOP and change something. And if you want, speculate on the reason.

You got me going enough here to try on one. :grin:

I think we have a chicken or the egg scenario here and "your" initial guess sounds as good as any. The pressure curve WAS longer on this bullet. But not for the reason you or I originally thought. I don't think it was a powder issue. I believe, as Leftie suggested, that it was a bullet hardness / sizing issue. Your picture gives me the clue. My only flaw in my argument will be because I am assuming this was a handgun.

I think the front was damaged mostly upon entering your bore because it had too much velocity for the hardness on that bullet design. If it had been caused by sizing in your throat, you would have seen it on the primer.

This caused a widening of the the front band. Had the bullet been made this way, it would have had a grease groove up there to handle it. But since there was no lube up there, other than what was left in the bore, this became inadequate. The 13 grains bullet on the right was already showing that you had a problem at that level and needed a harder mix or smaller size "for that design". And I have shot it at higher pressures than that with 20-1 and not seen that. The slumping then created more friction that may have slowed the forward motion of the bullet and we all know what that means.

So while the initial increase WASN"T greater than the 195 grain, the deformation of a wide band and soft bullet made the pressure curve much longer that distorted the bullet even more. Thank God your lube held.

Want to test it? Size down to .358 and repeat the test at 13 grains and go up and recover your bullets. I am going to guess that you won't see the same thing. Please report back if you do. If this was a rifle or a contender, I will be all wet. (won't be the first time trying guess)

[Black Jaque Janaviac]

Evidently posting pictures helps. Thanks guys. I'll get some picture of the Lyman 358430 soon.

I'll compare the length of the rifling surface on the fired bullets. I never thought to do that.

The alloy is wheel weights. I try to get them as soft as possible, so I anneal them in a toaster oven.

I purposefully soften them because I've had lousy results hardening them. I figured it might have had something to do with blowby - which I seemed to get a lot of. When I shoot softer bullets I get decent accuracy.

The 158RF/13 grs Lil' Gun is really accurate from both Black Hawk and M92 (photos are from a BH). The 158RF/17 grain load is fine from the BH. I put 5 of 6 in about 3 inches at 20 paces, offhand, onehanded.

I have had problems with the 158/13 gr load misfiring in really hot weather. That's what caused me to play around with the 158/17 grain load.

I recovered the bullets from a snowbank.

Incidentally though the 17 grain charge shows greater obturation, it has little to no signs of blowby. The lesser charge shows blowby at the base of the bullet. See below: 158RF/17 Lil' Gun on the left; 158RF/13 Lil' Gun on the right

[Black Jaque Janaviac]

Correct me if I'm wrong, but I think inertia is the same whether a bullet accelerates from zero to 1000 fps or accelerates from 1000 fps to 2000 fps. So if the bullet is already moving when pressure peaks, it should not have any difference on obturation. I think, now this is my rusty understanding of high school physics, the peak pressure occurs at roughly the same point of peak acceleration. There may be some slop there due to losses to rifling engravement or whatnot. But for the most part, the peak pressure is when the gasses are pushing the hardest on the bullet, and since A = F/M; the greater the F the greater the A. (F= force A=acceleration M=mass, as above).

I recovered some of these same 158s fired from my M92 and they exhibited MUCH more blowby. Their noses were also more deformed due to the greater impact velocity (different snow conditions too).

[Bass Ackward]

Correct me if I'm wrong, but I think inertia is the same whether a bullet accelerates from zero to 1000 fps or accelerates from 1000 fps to 2000 fps. So if the bullet is already moving when pressure peaks, it should not have any difference on obturation.

BJJ,

You keep going back to Physics. Are you still in school or what? :grin:

The NRA Lead Hardness Chart for Lyman#2 (15BHN) says that maximum pressure is 28,800 psi with no further information. When I take a 154 grain 30 caliber rifle bullet made of my ACWW + 2% Sn (14BHN), I can start it out with 4198 and have the mix fail at what my computer program predicts is 30,500 psi. When I change to RL19, I can take that same hardness and run it up to what is projected to be 34,000 psi before the mix lets go. My velocity is 500 fps lower with the 4198 than with the RL19 because the bullet is moving faster when the pressure finally peaks. My 2600 fps bullets with the RL19 show no signs of compression or deformation other than rifling. None. Nada. Zipo.

Handguns get so much trickier because you are sizing if your throats are bigger than your bore and you are doing it quite violently and then impacting your rifling at what amounts to about 700-800 fps depending on the actual travel length using 4227 and the 17 grain figures.

For something else to chew on:
Using 4227 for a guide,

13 grains is 17,183 psi that doesn't peak until bullet travel hits .378.

17 grains is 40,876 that peaks in .276 of bullet travel.

Pressure required to obturate ACWW is 17,238 psi which explains your gas cutting on the 13 grains. So you should not be seeing any signs of slumping on the nose or compression of the groove at all, yet you are. It should look like a perfect slug minus the rifling unless the damage is related to sizing / impact.

And since maximum pressure for ACWW is rated at 22k and change, your compression with the 17 grains I would say explains that. This is why they say to match your hardness to your load which in this case is the high end range for linotype which maxes out at 42k.

[leftiye]

Black, Just a note. I think B.A. has given you some primo stuff to ruminate on..

Whether or not this here helps here it is. It's not just how much something accelerates that determines the forces of inertia that are generated, it's also HOW QUICKLY! The shorter the time of acceleration (for the same increase in speed) the higher the forces of inertia.

[Black Jaque Janaviac]

Bass,

You were right! I checked some of the same bullets, charged with the same loads, but fired from my M92 rifle - almost no signs of compression! The lube grooves hardly fattened out at all.

Nope I'm not in school anymore. I'm just trying to use what I learned during school to figure out this bullet obturation deal.

[Bass Ackward]

Nope I'm not in school anymore. I'm just trying to use what I learned during school to figure out this bullet obturation deal.


BJJ,

Good. Now that you have an understanding, you are on a path to correction. If you need to!!!

I've got guns that like big bullets to stop leading. And those that want bore sized. Can't find a clue why. I just make a note of it. The logic is simply against "too" big of bullets on PB designs especially. When ever something happens that slows that bullet up, that base is saying get me the hell out of here. So I try to oblige. I mean you are corresponding with the guy who has spent a lifetime so far trying to use Lee Liquid Alox. As soon as I get it to work once, I try the same technique again and I am back at square one.

In the end, some of my best shooting stuff just defies logic and scientific fact. Sometimes there is a benefit to being ignorant and just accepting advice from Professor Gun and Mr Target. They seem to know best.

That is kind of a mot-ice operandi for me these days since I can't remember things I learned as fact or fiction anymore. :grin:

Every time is like the first time!!!

Thanks for following up.

[felix]

But you do too know why, BA! Checks tend to hold the base together mo'betta'. If you are going to kick those naked boolits as hard, they must be sized smaller for the same amount of obturation (for the same accuracy or whatever). ... felix

[Larry Gibson]

....... I'll blame that on Hodgdon thank-you-very-much. I don't know of ANY powders that can generate velocities equivalent to H110 with only 25,000 CUP!! I find that rather wierd and mysterious. So it's no big surprise that Lil' Gun produces some other anomalies.....

Obturation is based on the SPEED of accelleration in relation to the time/pressure curve of the load. It is not based on pressure alone. The faster and longer the bullet accelerates the more obturation there is. A load that accelerates a bullet as fast or faster as another load over a longer period of time will cause less obturation even if the pressure of the first load is equal to or less than that of the second load. It's all about the time/pressure curve. Understand that and your question is answered. There is absolutely nothing "wierd and mysterious" about it.

Larry Gibson

[Black Jaque Janaviac]

There is absolutely nothing "wierd and mysterious" about it.


Ok. If you say so.

[piwo]

Obturation is based on the SPEED of accelleration in relation to the time/pressure curve of the load. It is not based on pressure alone. The faster and longer the bullet accelerates the more obturation there is. A load that accelerates a bullet as fast or faster as another load over a longer period of time will cause less obturation even if the pressure of the first load is equal to or less than that of the second load. It's all about the time/pressure curve. Understand that and your question is answered. There is absolutely nothing "wierd and mysterious" about it.

Larry Gibson

I'm hoping I'm getting obturation on the solid base boolit I am shooting out of my flinter: a 410+ boolit pushed by 100 grains of FFFg. It's a 43+inch barrel, but the thing is, the bore at the breech end is wider then the muzzle, and even though one point of contact is = to the bore diameter at the muzzle, it's wider at the breech and consequently there would not be resistance at the initial explosion. Short of recovering a perfectly preserved boolit (impossible), how might I know/ increase the odds that I am getting obturation, thus sealing of the barrel during firing. Using the softest of leads..

Do I need to do a hybrid "patch" at the base or something along that line? Thinking if I did, I'd not get it loaded past the crown, but perhaps....

[Larry Gibson]

I'm hoping I'm getting obturation on the solid base boolit I am shooting out of my flinter: a 410+ boolit pushed by 100 grains of FFFg. It's a 43+inch barrel, but the thing is, the bore at the breech end is wider then the muzzle, and even though one point of contact is = to the bore diameter at the muzzle, it's wider at the breech and consequently there would not be resistance at the initial explosion. Short of recovering a perfectly preserved boolit (impossible), how might I know/ increase the odds that I am getting obturation, thus sealing of the barrel during firing. Using the softest of leads..

Do I need to do a hybrid "patch" at the base or something along that line? Thinking if I did, I'd not get it loaded past the crown, but perhaps....

The answer to your question; "Short of recovering a perfectly preserved boolit (impossible), how might I know/ increase the odds that I am getting obturation, thus sealing of the barrel during firing." is going to be answered in accuracy and/or no leading. If you are getting excellent accuracy and no leading I'd bet you are getting sufficient obturation. I'm assuming you workd up to the 100 gr load and it's accurate? If you are getting any leading then you are getting gas blow by and the obturation is insufficient given a good lube.

I shoot a 350 gr Maxi Ball out of my TC Black Mtn Magnum .50 cal cast of 1-16 tin - lead alloy. I use a 90(V) gr charge of 777 FFG with a felt wad under the Maxi. It runs 1525 fps out of the 26" barrel and I use that alloy to withstand the accelleration without excessive obturation. It shoots about 2 moa at 100 yards for 5 shots with no cleaning between shots. GOEX Cartridge BP at 120(V) gr runs 1464 fps with the same accuracy. With the 330 gr REAL bullet I must cast harder than lead to get any accuracy above 1400 fps so I cast them of 1-30 alloy. Soft lead Maxi's go south in the accuracy department right at 1200 fps and that's with 80 gr of real black powder.

Keep in mind that the barrel and depth of grooves along with how tight the bullet is in the barrel to begin with will all affect your results. However, again I would say that you are no doubt getting obturation with a 100 gr charge of black under a 400 gr bullet cast of soft lead. If it is not accurate perhaps you are getting too much obturation instead of not enough. If you've not worked up to that load start at 70(V) gr and shoot 5 shot groups at 100 yards working up in 10(V) gr increments to the 100(V) gr load. The most accurate group is going to be where obturation and all other factors are compatable. If the accurate load is below 100(V) gr with the soft lead bullet and you want to take it up a notch then try tin lead alloys of 1-40, 1-30, 1-20 and 1-16 with the 100 gr charge.

If you're wondering where I came up the 1-16 alloy it is the alloy that Frankford Arsenal came up with in the early 1880s for use with the 500 gr 45-70 bullet that proved the most accurate. That 500 gr M1881 bullet had to bump up (obturate) from .459" to fill the .461" +/- groove depth of the trapdoors over 70 gr of BP. I use the same alloy in my Blk Mtn Magnum as I do my Trapdoors because it works.

BTW; It is not the "resistiance" of the barrel to the bullet that causes obturation. It is the inertia of the bullet itself.

Larry Gibson

[MTWeatherman]

Lots of physics thrown around here...and no small amount of theories. BJJ, your understanding and memory of high school physics is outstanding.

Like JSH, seems to me obturation is a pretty hard thing to measure without actually getting inside the bore to see what happens...same with bullet acceleration where we have to rely on Quickload to give estimates. We do know that obturation is related to force (and pressure since it is simply force per unit area) because we in general observe it...raise the force or pressure (by changing bullets and/or powders to change the either the mass or acceleration in the Force=Mass times acceleration equation and we tend to witness an increase in visible obturation. However, things break down when we actually try to quantify the amount of obturation because we don’t (to my knowledge) have an exact equation that gives the true relationship between the varying lead alloy bullets we use and the acceleration (or rate of acceleration) involved. Since we can’t truly measure that obturation for our purposes, its hard to develop the equation (except perhaps in theory). How do we define obturation anyhow, is it the amount a given bullet should expand under a given force on its base in an unrestricted environment?....or how much it actually expands in the barrel or throat since it is in a constricted environment. Seems to me, for our purposes, all we can say is it obturates (fills the groove diameter if the bullet is undersized) or doesn’t (bullet is already groove diameter or we have gas leakage). With that definition, one bullet doesn’t obturate more than another...a bullet either obturates or doesn’t. That follows the defintion in the dictionary which is “the act of stopping up or closing”.

Details are lacking here, so I have to ask...how do you know you had less obturation with the 200 grain. Did the 200 grain have gas cutting? If your bullets are unsized and at .360, they well could be (and likely were) over groove diameter. Why would you expect or how would you measure obturation with an oversized bullet when your barrel was likely acting as a resizing die. Did you slug the barrel (or throat if a revolver)?

It is difficult to infer obturation differences between differently designed bullets because of differences of construction in any case. One bullet may more easily appear to show signs of obturation than another.

If you truly want to test whether or not the lack of obturation is related to the 200 gr bullet weight (or sectional density as in the initial question), try this. Cast up some 200 gr. bullets. Take several 200 gr bullets and carefully cut and file the nose down to achieve a 158 gr. weight...being sure not to affect the bearing surface. We now have very similarly designed 158 gr and 200 gr bullets (except for length and nose shape of course, however for our testing should make little difference). Fire the filed 158 gr bullets with 17 gr of Lil’Gun together with an equal number of the untouched 200 gr with 13 gr. Recover them and see if the 158 gr bullet wins the obturation test this time.

Good thread.......and welcome BJJ!

[piwo]

Larry,
The particular load I'm talking about gave me pretty encouraging results when I tried it a few weeks ago. A 410 grain boolit at @ 1450 fps: at 50 yards the 6 shots were all in the 5 inch black bull...... but at 100 yards, the last three boolits I had that day printed into a 1.8 inch group, after subtracting for caliber .542, left me with a @ 1.3 inch group at 100 for those three shots.. Nothing definitive, but something to be excited to try again... My rifle is just so "lose" at the breech, I just don't know if there's enough resistance to get the thing to seal properly. I guess if the accuracy is there, it doesn’t really matter, but I do think of it when laying in bed listening to the crickets at night…………
........

[Larry Gibson]

piwo

If you're not using wads under the bullet try some wonder wads. I have had the same excellent results with them as with my own cut felt wads. My own wads are thinner but I've noticed no difference in accuracy or velocity. I cut mine out of felt I buy at a fabric store. Made the cutter out of a section of old milsurp barrel. I put the cut wads in a baggie with some of my lube (olive oil/beeswax - 4 parts/5 parts) and put the baggie in the micro wave until the wads are saturated. I do the same for RB patches. The lube makes them stick to the bottom of the bullets so they load easily from a speed loader. Never had a maxi "walk the bore" in my rifle and I've handled her pretty rough when hunting. Sounds like you've got a good load going there. What Bullet, what caliber?

Larry Gibson

[leftiye]

Piwo, There's got to be SOME shock wave from that charge! Though it's much less scary than some of the other loads you've mentioned. Anyhoo, there's therefore a real high probability your boolits are "bumping up" to fill the grooves. Larry's idea about a wad is one that I've used to get my best accuracy. I cut them out of 5/16" feltan blue streaks (shotgun wads) and melt them full of wonder lube 1000. I call it a hydraulic gas seal because when fired the lube becomes liquid and seals around the boolit, stopping flame cutting. Another thang you might try (if you have a leftover piece of barrel) would be to cast groove diameter boolits and pre-engrave them. Less bumping up/ deformation.

[leftiye]

Black Jack, Easy answer to all of this is that if you were to keep your pressures below 1440 psi per Brinell Hardness Number, there would not be any obturation. The lead would be stronger than the pressure, and wouldn't deform due to pressure. Keep looking at what B.A. said about revolter boolits, and what was said about maybe slugging your barrels, and the revolter throats. Try sizing to plus .001" over groove dia in the rifle, and same +.001" over throat dia in the revolver. This would maybe show you if the boolits are too big (let accuracy be the telltale). And get some alloy to match your presure (18 for 25000 psi).

[jim4065]

............. And get some alloy to match your presure (18 for 25000 psi).

That's very interesting - I presume there's a comparison of BHN vs PSI for rate of flow (obturation rate) for alloys of lead. Makes sense - especially for the bullet swagers. Can you post a copy or send it in a PM? Thanks in advance.

Jim

P. S. Not wanting to get too far afield here, but are we dealing with a very viscous liquid, like glass? Never took metallurgy - this stuff makes my head hurt - but do all liquids have vapor pressure? Could sublimation (passing directly from solid to gaseous phases without becoming liquid) be causing my lead supply to dwindle alarmingly? Maybe a new tax deduction - Lead Depletion Allowance? :Fire:

[piwo]

Larry, Leftiye, please go to this thread: http://www.castboolits.gunloads.com/showthread.php?t=15636
and I'd like to continue on our discussin pertaining to our discussion. Even though steeped in obturation question, I don't wanna hijcack Black Jaque's thread. I posted some questions and musings there....:drinks:

[Larry Gibson]

Black Jack, Easy answer to all of this is that if you were to keep your pressures below 1440 psi per Brinell Hardness Number, there would not be any obturation. The lead would be stronger than the pressure, and wouldn't deform due to pressure.....

I don't think the pressure vs BHN is exactly correct. I am successfully driving a RCBS 30-150-F with a BHN of 15 to 2500 fps and maintaining sub 2 MOA at 100 yards. This is out of a .308 Win with 1-14" twist. According to the formula I shouldn't exceed 21,600 psi. I'm using 39 gr of 4895 and would bet it's a little higher psi than that. I and others also push cast bullets in .22 Hornets to 2500 fps or so with excellent accuracy, they have 1-14/1-16" twists. And as of recent I've pushed some 160 gr 358156 cast of WWs to 2300 fps with very good accuracy out of my 1-16" twist .35 Rem barrel. I'd venture a guess that those also exceed the formula's limtation. I believe the formula was developed as best accuracy comes around 1700-1900 fps with most 1-10/1-12" twist .30 cals. The formula seems to work there but maintaining the best accuracy with normal cast bullet designs is a function of RPM instead of psi given a certain alloy hardness.

Larry Gibson

[Scrounger]

I think gas checks do at least a little to skew the BHN/pressure rule. What is the BHN of the gas check material?

[leftiye]

Larry et. al, The reference to BHN # 18 was a hardness that should put the strength of the alloy above the point where a pressure of 25000 would be able to overpower it and deform it. It is probably a little higher than necessary, and there are all kinds of people's personal ideas as to how to use this calculation. Some like to stay 10% below the pressure that will "bend" the metal, some look at it as a way to get moderate obturation and use pressures 10% above the modus of elasticity (If I've got the correct term). Others just plain ignore it and depend on the fit to the gun to avoid accuracy robbing deformation. The attempt was to show a way to take obturation out of the picture for testing purposes. However, with soft alloys this approach may be the cat's meow for preserving accuracy.

[Larry Gibson]

leftie

Ok, that sounds good to me.

Larry Gibson