Cast Boolits Oversized

[rodwha]

I've always seen that lead bullets are 0.001" over bore. Yet with percussion chambers it's often seen the produce a projectile well under bore. Some ream their chambers to bore or over, but many, including match shooters, claim it is unnecessary (and foolish) saying that obturation fixes this.

I certainly cannot say, but it seems some of these undersized chambered guns suffer quite a bit and aren't very accurate because of those chambers. But then the same can be said for muzzleloaders requiring a felt wad to seal the bore well enough.

Why do lead bullets need to be oversized for most applications, if not all?

[44man]

It is to take rifling twist without skid. A tad over groove provides the seal and as long as it does not skid past the base band all is good.
Smokeless has a different affect then BP plus BP guns use pure and BP will still bump them up.
I have found bump up is not good with smokeless, better to seal at the start. Fast powders are harder on a boolit too, instant pressure. 17,000 psi applied right now is still harder then 40,000 psi spread an inch or two.
Big problem with a C&B is twist rate, most not right for a RB, too fast, so guys use a boolit that reduces the charge and velocity to put you in the same boat.
A RB in a C&B works better with a reduced charge and filler or wads. I hunt with mine so want a full charge but it reduces the accuracy distance a great deal. There is a trade off.
But a modern revolver must have throats larger then groove, harder boolits at least groove to a little over.
Harder to resist seat sizing and take rifling for the seal.
I have extreme accuracy with a .430" boolit with .430" groove and .4324" throats in my .44. A .432" boolit is accurate but I can't tell if it is better. The .430" has done 1-5/16" at 200 yards. The .432" does 1-1/4" at 100. I don't go to extremes.
Now a RB in a ML rifle needs fit. I don't go with small balls unless rifling is shallow or my pants are tight! My .54 takes a 535" ball and a .022" patch, the ball must be engraved .005" at grooves, never mind lands. Even a Minie' ball in a musket must be a hard thumb fit and not depend on expansion. Had too many here that would not hold a 4X4 cardboard at 50 and after lapping the molds, would hit 200 meter targets.
Fit is always king!

[DougGuy]

Since the rifling grooves cannot be pre-machined into the sides of the boolit, it is a fair truth to say that SOME obturation MUST occur. Nothing shoots good without some sort of seal in the bore be it a wad, sabot, or a boolit.

By the time a revolver fires the boolit out of the cylinder and it encounters the forcing cone, it can be distorted slightly and if it is too close in size to groove diameter in the bore, may not fit in the bore tightly in the area that was reshaped by the contact with the forcing cone.

A larger boolit would have enough excess size to withstand any amount of re-arranging that the forcing cone may do to part of it, and then still seal in the bore effectively.

Obturation will cause the boolit to be pushed outward as it is pushed forward, until the outward movement of the lead alloy is contained by either the cylinder throats, or the bore itself. For this to occur, the alloy the boolit is cast from must be soft enough to allow the available pressure from ignition to cause it to change shape, to fill in the shape of the container that it is contained by. In this case, it would be a cylinder throat at first, and then the bore. Alloy that is harder, will require more pressure to cause obturation, up to a point where the alloy is simply too hard for obturation to occur at the maximum amount of pressure that the firearm is safe with.

There are formulas that will match the hardness of any alloy with the velocity and pressure it is fired with, so you can figure out how hard your alloy should be. Some applications will require dead soft lead, most will require a mixture of pure lead and a harder metal such as linotype, tin, etc.

As 44man said, fitment is KING. Whether you do it mechanically, or your gun does it with pressure from burning powder, fitment NEEDS to occur for the gun to shoot accurately. For best fitment, the boolit generally needs to start out at the very least the same size as the groove diameter in the bore, preferrably .001" to .002" larger.

If cylinder throats or barrel throat is sizing down the boolit upon firing or preventing it from being chambered before firing, then fitment is hindered and the boolit cannot be delivered to the bore at an appropriate diameter and accuracy will suffer. Obturation many times saves the day with tight cylinder throats, and often the shooter never even knows the throats are too small!

[OS OK]

What a great discussion, it seems that early of the morning our brains are engaging well.

[MrWolf]

Agreed, good explanations. Thanks.

[DougGuy]

I have extreme accuracy with a .430" boolit with .430" groove and .4324" throats in my .44. A .432" boolit is accurate but I can't tell if it is better.

Fit is always king!

I wonder, if your loads with the .430" boolits were fired in a cylinder with no barrel in front of it and then recovered, what their diameter would be. As hard as you like them, I am willing to bet they are a good deal larger than any .430" when they exit the cylinder throats. Just sayin'

[longbow]

I am a believer in shooting boolits that are at least 0.001" larger then groove diameter... this being in rifle since I don't reload for handgun.

The worst gun I have for leading (used to be) was a Marlin 1894 in .44 mag. Two main issues: "fat" groove diameter of 0.4315" (when compared to .44 mag handguns) and tight spots in the bore. Most generally available moulds cast to suit a nominal 0.429"/0.430" for handguns so are undersize for rifles (SAAMI groove diameter spec is larger for rifles) and in my case anyway, resulted in bad leading before I realized my Lyman 429421 was not fitting my fat Marlin barrel and was casting at 0.429" using wheelweights.

The boolits did not obturate to fill the bore even when cast of ACWW and recovered boolits showed significant gas cutting. I solved this by a) buying MIHEC and Accurate moulds that cast at 0.433"+ and b) hand lapping the tight spots out.

Now having said that, I was doing a boolit penetration comparison with a variety of boolits and that same Lyman 429421 was collapsing at the lube groove over a max. book powder charge. So it should have obturated to fill the grooves if it is collapsing under the pressure but did not. That makes me wonder if the gas cutting starts in the throat not that Marlins have much of a throat.

The fact remains that the boolits were actually distorting under pressure but gas cutting was still occurring with the undersize boolit but does not with a boolit of a thou or so over groove diameter.

As 44man says BP also has a more abrupt pressure curve which will generally bump boolits/balls up at firing, especially if they are cast from soft lead, which may allow for somewhat undersize boolits and still provide accuracy. Smokeless powder does not have that abrupt pressure curve so is not known for bumping up boolits.

And as 44man said "Fit is always king!" I can't see how using a boolit that is a thou or two over groove diameter would adversely affect accuracy where an undersize boolit may or may not bump up enough to fill the grooves and if hard cast will not likely bump up at all.

My take anyway.

Longbow

[44man]

I wonder, if your loads with the .430" boolits were fired in a cylinder with no barrel in front of it and then recovered, what their diameter would be. As hard as you like them, I am willing to bet they are a good deal larger than any .430" when they exit the cylinder throats. Just sayin'
Yes, I suppose even the hardest lead will expand, from the tremendous pressures. Jacketed will also upset. It is that I depend on straight starts. Getting sized in small throats does not make good fit after in the bore.
Guys don't understand light loads have very high pressures. Look what 100# from a compressor can do.
The worst is always sizing smaller then groove in throats or making a boolit larger then throats.
You really can go under throat size if the cylinder clocks. The hardest boolit might reach throat size but it is of no use unless the ogive pulls the cylinder to alignment.

[PositiveCaster]

Forty years ago when I began shooting cast in my RBH .45LC I noticed leading with ACWW metal. Recovered boolits exhibited gas cutting as would be expected. I don't remember the exact charges of Unique I was using, but I believe that I began with 8.0 grains and accuracy was mediocre. I increased the charge 1/2 grain at a time and accuracy improved; at the same time leading decreased. When I reached 9.5 grains all leading stopped and recovered boolits showed no evidence of gas cutting.

Much has been published in the last few decades about bullet fit and leading, the first achedemic one I read being by Dave Scoville of Handloader magazine. To eliminate leading the alloy hardness must be matched with enough pressure to obturate the bullet. Soft boolits require less pressure than hard boolits. Scoville published a formula to estimate the pressures needed for different alloys.

To say that a bullet won't expand to seal the bore ignores the affect of pressure and hardness. With enough pressure a lead bullet's yield strength will be surpassed and the bullet will obturate. It is well-known simple physics.



.

[longbow]

I believe there are some misconceptions regarding lead, yield strength and obturation. A lead alloy of 8% antimony heat treated can reach 12,500 PSI yield strength. That is the highest yield strength of lead alloy I have found though possibly linotype or others will exceed that some. However, the yield strength of most lead alloys is lower than chamber pressures generated by even low pressure loads. However chamber pressure does not produce the same stress in the lead. It is bore friction and momentum that produce the stress in the lead boolit as resistance to movement.

The base of the boolit sees the highest stress as it is being pushed on directly by the chamber pressure and it is pushing on the rest of the boolit mass and fighting bore friction so the boolit will upset most at the base with decreasing upset towards the nose.

I think many misinterpret the calculations as determining the required yield strength of the lead but it is the maximum chamber pressure suitable for that the hardness of lead being calculated... at least with the calculations I have seen as results far exceed the yield strength of any lead alloy.

"Fit is always king" applies always in my experience. Fit can be obtained by using oversize boolits or by using alloys soft enough to obturate. I think hard boolits that do not obturate can work just fine if they are oversize to start with so they seal well.

In a revolver that would require cylinder throats larger than groove diameter to allow those oversize boolits to be used. As for BP revolvers, generally hard alloys are not suitable so soft lead it is.

Longbow

[OnHoPr]

Since I have been on this site and shooting cast boolits I have had a problem with understanding the term obturation. I might see this in a number of low pressure loads like with a 38 special and soft boolits or bevel base wad cutters. But, most everything I see whether it is with a milsurp, win 94, or a enlarged bore Marlin is that most boolits are cast overbore dimensions with the Marlin being noted to cast larger than most for the micro groove barrels and possibly with a little bit harder alloy unless the caster/shooter is a bit more advanced. How can a boolit have obturation or bumped up during ignition and its path down the barrel when it is already cast over sized for the bore and possible lead. It sounds like the term extrusion would be more appropriate for what the actual boolit is doing. It is not filling the bore it is being shaped to the bore dimensions. Now if the boolit is to small and won't obturate or get bumped up it seems that leading occurs. If the boolit is to large and possibly to soft a yaw or deformed boolit is created during the extrusion down the lead and barrel leading to inaccuracies. Here is where the term proper fit comes in where the boolit gets centered properly and squeezes into the bore just right on ignition for the best shape and accuracy. Now, there also might be a consideration about what these pressures put on the boolit which then puts certain pressures to the side of the bore preventing skidding. Obviously, the harder the alloy the easier for the novice/average caster and the more advanced casters can go with softer alloys when something other than a 38 special plinker load is made with a bevel base WC.

[rodwha]

Judging by what's been pointed out I'd say that the reason modern arms about demand an oversized lead bullet is the typically harder alloy coupled with the slower pressure buildup of smokeless compared to the BP arms.

My ROA has chambers 0.001" over groove, whereas my Pietta Remington NMA had 0.446" chambers with 0.452" grooves. Fly reamed my chambers to 0.449".

I've wondered if it would be worthwhile to open them up further as I can't say I can tell an appreciable difference. But then it shoots fairly well anyway. It doesn't give me as tight of a group compared to my Ruger but it's not that far off.

I've often read how the Pietta Remington Pocket has extremely small chambers and many state the accuracy is horrible.

[44man]

I believe BP has a lower pressure build up then smokeless, it does not all ignite at once. Experiments we did with overloads showed longer barrels are needed to burn it all. Even then we shot plugs of powder out the muzzle to burn in the grass, Pyrodex really good for flares!
Chronograph showed as the charges were increased velocity would drop a large amount as more powder was pushed adding to boolit weight.
It is different with an inline that uses plugs of modern subs. They have a hole in them and use hot shotgun primers to put flame all through the plugs.
BP has a limit to pressure and velocity so twist is more important with a RB or boolit. As twist get faster the boolit length is longer, also the new powders burn different. There is a vast difference in brands of BP too with Swiss the fastest. Then all the granulations there are.
Then you need to look at powder compression, all is not equal. Pyrodex needs some but too much goes backwards. Loose 777 wants NONE at all so in a cartridge you can't get enough in. Swiss can stand more then is said about it, it can take as much as Goex.
The word obturate means to SEAL so it is used in the wrong context. A fit boolit obturates without expansion.
There are evil things with too soft a boolit, slump to change the entire shape you spent hours making and buying certain molds for and skid in the rifling. Keith boolits have a problem guiding a cylinder because the shoulder will get wiped on one side that hits the cone first. Most revolvers are not perfect, even the most expensive so you need some play to let a boolit get alignment. My work with a Keith has shown as I made the alloys harder, accuracy peaked at 28 to 30 BHN.
Lube purge fascinates me too since a fit boolit will push everything out of the bore every shot. It is what a good lube is for, to keep fouling soft so it never builds up. The boolit is a cleaning jag.
Very hard lubes are no good in revolvers or too slippery either. Hard can stay in portions of the grooves and make a boolit out of balance. Slippery can break case tension early from primer pressure alone. That changes air space shot to shot.
Case size determines the primer used and even the .44 mag is too small for a mag primer. The .454 should use a LP mag primer, NOT a SR.
If you bring .44 loads here to shoot, I can tell you what primer you used.

[44man]

One other thing to consider. Shoot a boolit through a tight throat and expect it to expand again in the bore to seal is slump city. Pressure from fast powder is already dropping with no more push left.
In all these years I have NEVER said to soften a boolit. Fix the gun and get fit. Get even case tension shot to shot. Test primers and lubes. Stop reading gun rags.
Doug can fix the gun if throats are wrong. Lloyd never says wrong either.
You can't work around things. The revolver is the hardest gun ever to get right but they can be amazing. The only revolvers I have dumped has been from grip fit to me.

[rodwha]

Triple 7 only calls for very light compression in cartridges. It's not the same otherwise. For consistency I always seat my projectiles quite firmly no matter which powder (Triple 7, Olde Eynsford, or Pyrodex) I am using. Some claim compressing T7 gives erratic pressures, but if this were the case my POI and group sizes would change, and they don't.

There isn't a whole lot of meat between the chambers left, which has left me hesitant to open them up any further. It looks too thin as is.

[44man]

777 needs touched and no more. I have a tool when loading Pyrodex to get the same pressure. Spring loaded on the ramrod.

[longbow]

Not sure about really fast powders like Bullseye but generally speaking smokeless powder builds pressure slower than BP. The pressure generated is usually higher than BP but slower to peak. At least that has been my understanding. However, an internet search did not reveal much in the way of pressure curves for BP and smokeless powder comparisons... in fact not much about pressure curves at all.

I did find one graph that illustrates what my understanding has been anyway (attached).

180028

This is for shotgun and BP vs. and unidentified smokeless powder but should be similar for rifle or handgun.

BP cartridge shooters that use paper patched boolits patched to bore diameter depend on the bump up (obturation) of the much undersize boolit to seal the bore and it works. The fast pressure rise of the BP + soft lead boolit results in the bump up to groove diameter as the boolit leaves the cartridge.

Maybe someone has better pressure curve comparisons for a variety of powders and firearms and can elaborate some.

Longbow

[DougGuy]

There isn't a whole lot of meat between the chambers left, which has left me hesitant to open them up any further. It looks too thin as is.

That would have been my next question. I can use the Sunnen hone to take them out to whatever diameter, but when Colt made those originally, they made them with the smaller cylinder and Pietta copied the original.

The web thickness between charge holes would be of major concern.

[OS OK]

longbow...re: powder curves etc...


'The Wizardry of Propulsion' (http://castboolits.gunloads.com/showthread.php?297867-The-Wizardry-of-Propulsion)

This is from a Speer write-up from 1966...I think, but it's illuminating in the least...

[rodwha]

According to Hodgdon it needs firm compression in anything but a cartridge.

For a cartridge:
"Loading density should be 100% with light compression not to exceed .100"."

For anything else:
Seat the projectile firmly against the powder.

[rodwha]

This is my Pietta Remington NMA next to my ROA cylinder.

http://tinypic.com/r/24yw8at/9

[longbow]

OS OK:

Interesting stuff there. Only for smokeless powder though. What I am looking for is a comparison in pressure curves between smokeless powder and BP. Of course there are a variety of smokeless powders and BP granulation so burn rates and pressure curves will vary but generally I think it is accepted that BP will bump up a boolit to seal the bore before any gas cutting occurs where smokeless will not.

It would certainly be interesting to see the various pressure curves to compare, or even test info in a strong gun where BP and smokeless could be compared for boolit bump up, accuracy, gas cutting.

I looked for quite a while but only found the one graph showing BP vs. smokeless powder and it does indicate a more immediate pressure increase for BP.

I am surprised there is little info on smokeless powder though there were a few graphs of pressure curves but very limited powders shown. At least from what I could find.

Longbow

[44man]

BP could I guess, just strange results with it and how too much and compression affects it. I have some paper work somewhere talking about how newer BP should have new charts because it has been made better since the old days.
One experiment of interest was going from soft, lead/tin to WW boolits. They did not shoot good at all and they leaded my bore. They started over size too so that is strange.
Now something about the Rem revolver, friend has one and the chambers are close together, so close the jet from the gap will cut a boolit nose badly, the next chamber is almost under the gap. Going to the RB was much better with no damage seen but it did blast out all the lube.
I also found a BPCR lube is much better over a ball instead of something like Crisco, even in the ROA.

[OS OK]

OS OK:

Interesting stuff there. Only for smokeless powder though. What I am looking for is a comparison in pressure curves between smokeless powder and BP. Of course there are a variety of smokeless powders and BP granulation so burn rates and pressure curves will vary but generally I think it is accepted that BP will bump up a boolit to seal the bore before any gas cutting occurs where smokeless will not.

It would certainly be interesting to see the various pressure curves to compare, or even test info in a strong gun where BP and smokeless could be compared for boolit bump up, accuracy, gas cutting.

I looked for quite a while but only found the one graph showing BP vs. smokeless powder and it does indicate a more immediate pressure increase for BP.

I am surprised there is little info on smokeless powder though there were a few graphs of pressure curves but very limited powders shown. At least from what I could find.

Longbow

I'm sure there is volumes written complete with charts and graphs...but, I think it's proprietary and they don't want to share.
The best we get is generalizations and rules of thumb so to speak.

[longbow]

Well, I guess it all costs money to do the testing and then write ups and none of it is essential to the reloader so no point for powder companies in spending the money. It certainly would be interesting to see though. Likely would prove and disprove many "rules of thumb".

In the end all that matters is that we have safe reloading info to use but I am a detail kinda guy and like to know the hows and whys.

In this case the issue is boolit bump up and alloy hardness for sealing the bore.

I wonder if anyone shooting BP using bore size boolits in muzzleloaders or cartridge guns has tested to find minimum powder charge to give boolit bump up. For that matter I wonder if anyone using a modern gun suitable for smokeless has done a comparison between BP and smokeless powder to see if bore size boolits can be bumped up using smokeless. Faster powders like Unique, which is suitable for say .45-70 would be interesting to try.

But I digress. Kind of getting off topic here.

Longbow

[44man]

Bump up is deformation and I don't like it. I want to recover boolits that are the same as I cast except for rifling marks. I want to see the base band with marks NOT larger then rifling. OK with skid at the front but must stop at the base.

[Thumbcocker]

I have had good luck with a lube cookie under the ball in a 58 Remington and an Old Army.

[longbow]

No argument on bump up here. I can't see the point unless you are paper patching for BP cartridge and even then I am not sure quite why when everyone seems to blow tube and/or wipe between shots however, it works for them.

For smokeless powder use though and normal cartridge loading a boolit at least 0.001" over groove is far preferable to counting on bump up to make the boolit fit the bore. My view is get it all right before the boolit is even loaded then you know what you have... no bump up required.

Longbow