Defining boolit "fit"

[Bret4207]

One question for you however, since fit is King. Is there a thread here that in which the process of bullet fit measurement and determination is brought forth?

Boy, I wish there was. Correct fit is what the gun wants, I don't know how to explain it much more than that. I'm hesitant to try and define "fit" as I think it might mean something different to me than to others, but both of us may be correct. So keep an open mind on this.

In extremely general terms and IMO only most guns seem to respond well to a boolit that is about as large as will chamber easily. It usually seems best to err on the large side, undersized boolits (I include those that depend on obturation or "bump" for fitting) never seem to shoot as well as those that are closer to the right size in the first place. But that's not a hard and fast rule. And I can't say that any one formula ("...size .001 over groove size") works either. There seem to be odd duck guns that like a boolit at groove size and many others that shoot better with a range of boolits well over groove size. Boolit design can play a part in this too and of course there are limits to just how much we can do to affect the fit. In the end the powder/pressure combo is the final part of fitting a boolit. The barrel is the ultimate sizer.

So I suppose it's best to say that ultimately "fit" is finding the balance point or the happy combination of boolit size/design/pressure/temper/alloy (and probably a couple things I missed) in a particular gun with those particular components. In more basic terms "fit" is finding the general area the particular gun like it's boolits sized to.

I tend to like using a boolit as close to "as cast" as possible. Every sizing operation we do adds the chance of damaging the boolit, so I tend to be in the "leave well enough alone" camp. If I get problems at cast size, it's quite simple to try sizing down in steps and to see if that helps. You can also try different seating depths as that seems to affect the pressure curve and resulting fit. Sometimes a different powder will help, especially as you raise the FPS- too fast a powder can really ruin your day. I try not to jump to changing alloy. I prefer to use what I have a lot of that means WW alloy. There is a limit to what any alloy will handle pressure-wise, but WW with very minor additions of tin have worked for me up to 2200fps in rifles and 13-1400 in revolvers. No matter what alloy you use you still have to have fit the boolit to the gun.

Sometimes fit is simple and the first load you try will be a winner. Other times it seems that particular gun might hate a design of boolit and nothing makes it work. Some guns are just cast friendly, others hate cast of any kind. I don't know why, it just happens. When you get a good shooter and you want to really fit the boolit to the gun you have to move slowly changing one thing at a time and keeping good notes. NEVER change 2 things at once if you want to know WHY and WHAT worked or didn't work. You may see a trend that sticks out or over a number of experiments see a pattern that gives you insight into what seems to work in general terms. If you do, please share the info.

There are others here who have a much better handle on explaining things than I do. So I'm going to cut and paste this into another thread of it's own.

[Gohon]

Decided to continue testing to see how fast a bare bottom (non gas checked) bullet could be driven without a leading problem. Gun used was a 1894C, cast bullet was 158 grain LFN bevel base cast wheel weights with 2% tin from a Lee mould at 12 BHN, shot as dropped from the mould at .360 diameter. Three groups of ten rounds each were made up using 2400 powder with charges of 14, 14.5, and 15 grains. Set up 15 half gallon water jugs filled with water at 50 yards to test for penetration and look for any expansion. That didn't work at all. For some reason the bullet would not travel straight through the jugs and either in the 3rd or 4th jug the slug would exit to the left side. No slugs recovered and no idea what penetration would have actually been.

Next step was to check for accuracy so I just set up at 50 yards. The 14 grain charge grouped at 3 inches. The 14.5 grain charge grouped at 3.5 inches and the 15 grain charge grouped at 1.25 inches. None really great but the 15 grain charge showed promise for deer and coyotes at 100 yards.

Then it was on to the chrony and here I got a surprise. I had been using book data figures for speed but the chrony showed them to be way off. The 14 grain charge clocked at 1736 fps average of four shots, the 14.5 charge was 1804 fps and the 15 grain charge was 1843. These were a lot faster than I expected and a lot faster than some claim a bare bottom cast can be driven without leading.

So that was the next step..........checking for leading. I dropped out the lever and pulled out the bolt and looked down the barrel. I could see nothing but the usual burned powder residue. I then ran a single pass with a bore snake and checked and there was no leading what so ever. Some would tell you this is not possible with soft bare bottom cast bullets, especially a bevel base but there it is. If I decide to use this for any hunting I plan to drop back the the 13 grain charge of 2400 as that was my best accuracy load and still ran just over 1500 fps during earlier tests.

Bret, I need to pick your brain a little with a question. I posted the above quote on another site yesterday in the bullet test section. The underlined section in the first paragraph is what I'm trying to figure out. I've proven to myself the importance of proper bullet fit and that bare bottom bullets can be drive faster than most think without leading the barrel but I can't figure out why the slugs exited out to the left instead of going straight through the water jugs. I know the slugs weren't key holing because on the targets they were perfectly round holes, at least they seem to be. The only two things I can come up with so far is the spin of the bullet as it entered the jugs were affecting the bullet path or there was just enough yawn to the bullet that the path was affected upon entry. What are your thoughts and have you experienced this before?

[Cherokee]

I'm not Brent - But my opionion: Deflection, water does not compress. Its like hitting brush. Now Brent will give us the correct answer.

[runfiverun]

look behind the berm sometime, most of the spent boolits will be off to the left side.
at about a 45* angle to almost right inline.
i think it's tumbling with the rear of the boolit taking over and being tossed to the left.
because of the right hand twist.
i doubt any of your holes were square.

[thx997303]

Gohon, may I ask why you have to ask that question here?

Might I suggest the PM function?

Your question is more about terminal ballistics than boolit fit.

[Gohon]

Gohon, may I ask why you have to ask that question here?

I thought the posted quote went along with Brets comment about proper fit of cast bullets and how much it does matter. The exiting of the bullets in the water jug was a side question.

runfiverun, at fifty yards the bullets showed no sign of tumbling on paper targets so I know they entered square, or at least appeared to.

[Leftoverdj]

A bullet fit is the dance you do when you drop a freshly cast bullet into your shoe.

[JSnover]

[B]
The barrel is the ultimate sizer.
...

I tend to like using a boolit as close to "as cast" as possible. Every sizing operation we do adds the chance of damaging the boolit, so I tend to be in the "leave well enough alone" camp.

My thoughts on that; The bore and the grooves may not be concentric. When gunsmiths used lead laps to lap barrels they were unable to agree on whether or not to index the lap, i.e; should they turn it one groove partway through the job to 'even things out' in case a bore was eccentric or the rifling hadn't been cut to the same depth all the way round? Some would always do this, others would never do it.
To size the boolit perfectly round could be pointless in some barrels.

[geargnasher]

Boolit fit verb (without object): The comparative dimensional relationship between a Boolit and the portions of a gun it contacts during firing.

Good boolit fit is what is achieved when, all other factors having been eliminated, the reasonable velocity and accuracy limits of the firearm/boolit design are achieved without undesireable bore fouling.

See also, "Effective lube" and "Proper alloy" as well as "Correct Powder" and a thousand other things that make a gun shoot straight or not.


Like you say Bret, very subjective. In revolvers the best size for a boolit is usually relative to cylinder throat size, and many rifles it is barrel groove diameter, even if the case necks have to be turned down and expander balls enlarged to enable loading and chambering of a sufficiently large boolit to satisfy the appetite of the particular gun.

You really know how to open up a can, don't you? [smilie=l:

Gear

[rhead]

Gohon: is it possible that the jugs were not presenting a square face to the bullet at impact? Any deviation from a 90* angle would be multiplied due to the increased resistance of the water.

[Gohon]

rhead: it is certainly possible but to try and avoid that I had laid out a long 2x4 and placed the jugs against the 2x4 for alignment. However your question caused me to realize something I hadn't thought of before. As you know water and milk gallon jugs are square on the lower half but the handle portion is recessed in a corner of the upper half. I had placed all the jugs in perfect alignment with the handle portion in the left forward corner which would be about the ten o'clock position. That resulted in the water column being taller on the right side by a few inches than the left side.

This question though not directly related to bullet fit was something that occurred while working on bullet fit and it simply baffled me and caused me to wonder what this particular load or loads would do on real game. Plus I thought the test results of over sized bullets I was testing in my quote were in line with Brets statements. I think maybe in a indirect way you have helped me solve the puzzle.

[Bass Ackward]

Bullet fit? The definition is easy. It's the doing that is difficult.

Definition: Bullet fit is defined as that state required under a certain set of mechanical specifications governed by a certain pressure level and hardness to establish and maintain perfect bore alignment and rotational center into and up the bore. This is necessary so that the base of your bullet exists perfectly perpendicular to the crown or muzzle. This defines a perfect launch.

From that you can see how the requirements for bullet " fit " can / will change depending on all the variables from throat shape, size, jump to the rifling, bullet design and pressure intended to how hard your bullet is to establish and maintain that alignment.

Let a bullet get out of alignment, or fail to hold bore center at ANY velocity level and you will become a believer in the matching bullet to twist rate or the RPM theory. Either one means you failed to maintain rotational center for what ever reason.

[felix]

Great expose, John! Best explaination yet. ... felix

[montana_charlie]

One question for you however, since fit is King. Is there a thread here that in which the process of bullet fit measurement and determination is brought forth?
Yep. There have been threads where bullet fit was the main topic.
In it (or, those) it was pretty well agreed that 'fit' involves quite a bit more than 'diameter'.

You could cast a straight cylinder that is '.001 over groove' and you would have acheived correct fit, if diameter was the whole story.
But, each chamber has an internal topography that will hold a certain design in alignment with the bore, while a different 'shape' will exit the muzzle with the nose spiraling around the line of flight...like a football that wobbles a little on it's way to the wide receiver.

Some chambers are cut to accept long, slender noses, others can only accomodate a stubby bullet that barely pokes out of the case.
When differences are that extreme, diameter is a very small part of the equation.

Unfortunately, bullet fit requires discussion of chamber shapes, and that means using 'chamber terminology'. People have varying definitions for those terms, so the discussions always degenerate in arguments over what a 'leade' is or what is the actual meaning of 'throat'.

The 'fit' discussion never seems to make it to a conclusion or consensus.
CM

[dromia]

Bass has given an excellent definition encompassing the complexity of variables necessary for cast boolit accuracy.

What I know is that for any rifle there is a set of measurements of the chamber, throat and bore/groove that need to be established first.

Having done that you then want a boolit that is as supported as much as possible within the physical dimensions of chamber, throat and bore/groove to ensure as concentric alignment as you an get with the boolit and barrel. This is to ensure minimum deformation to the boolit upon firing and its travel into the barrel so as to be fully supported and engaged with the rifling gearing.

Once you get the best boolit fit you can then all the other variables of alloy, pressure, boolit weight, start velocity et al come into play and can be experimented with.

So with fit being king if you get this to be the best you can then it is a variable less to consider when devloping a load.

How do I know this? Well I've had boolits made to fit various chambers on specific rifles, sometimes ther've been compromises due to the nature of the rifles, like too short a throat on a new barrel, no wear means that you have to live with a lighter boolit than you'd wished for.

However in all cases where I've got good boolit fit for a specific rifle, bespoke or serendipity/coincindence then load/velocity accuracy development has been so much easier.

So for me boolit fit is king as best as you can get it. It is the foundation that all other load development is based on, at least in my approach to casting, loading and shooting cast boolits.

I'm sure there are far more scientific approaches to this but practically and tangibilly for me, good boolit fit is my proven starting point with cast boolits.

Let the contrary come forth. :grin:

[Bret4207]

Bret, I need to pick your brain a little with a question. I posted the above quote on another site yesterday in the bullet test section. The underlined section in the first paragraph is what I'm trying to figure out. I've proven to myself the importance of proper bullet fit and that bare bottom bullets can be drive faster than most think without leading the barrel but I can't figure out why the slugs exited out to the left instead of going straight through the water jugs. I know the slugs weren't key holing because on the targets they were perfectly round holes, at least they seem to be. The only two things I can come up with so far is the spin of the bullet as it entered the jugs were affecting the bullet path or there was just enough yawn to the bullet that the path was affected upon entry. What are your thoughts and have you experienced this before?

I think you have a good idea with the shape of the jug, maybe it combines with the twist, but that sounds like the culprit to me.

Excellent results though! That sounds like some pounder loads at those speeds. Good on you!

[Bret4207]

You really know how to open up a can, don't you? [smilie=l:

Gear

HAH! This is nothing. Did I ever mention I thought most 9mm's were way bigger than they need to be?

That thread drew BLOOD!

I've been saying fit is King for months, I knew sooner or later we'd need to have this discussion. There's no need for spittle on the screen, we can do this like bog boys.:drinks:

[Pat I.]

I'll go with Dromia's explanation of bullet fit, it's all in the throat.

[Bret4207]

Bullet fit? The definition is easy. It's the doing that is difficult.

Definition: Bullet fit is defined as that state required under a certain set of mechanical specifications governed by a certain pressure level and hardness to establish and maintain perfect bore alignment and rotational center into and up the bore. This is necessary so that the base of your bullet exists perfectly perpendicular to the crown or muzzle. This defines a perfect launch.

From that you can see how the requirements for bullet " fit " can / will change depending on all the variables from throat shape, size, jump to the rifling, bullet design and pressure intended to how hard your bullet is to establish and maintain that alignment.

Let a bullet get out of alignment, or fail to hold bore center at ANY velocity level and you will become a believer in the matching bullet to twist rate or the RPM theory. Either one means you failed to maintain rotational center for what ever reason.

See, I told youse there were people with a better handle on explaining things! We seem to all be in agreement, so let the juices flow a bit and lets see if we can get a good thread out of this.

[Bass Ackward]

I'll go with Dromia's explanation of bullet fit, it's all in the throat.


Pat, not to be disrespectful, but based upon our RPM discussions of the past this is a key point. And in your position, it is critical that you understand the point.

Yes. And no. You can fit a bullet perfectly in the throat and lose it up the tube. When you do, you have your RPM limit. Besides, a throat is ever wearing. (changing)

Especially, if the chamber is not true with the bore and requires it to turn while under pressure to advance. Then if your bore's dimensions allow gas to pass or can't hold center, or you pass fouling that does the same exact thing, then fit in the throat is wasted.

The RPM limit that we all experience is determined by what happens the entire way until the launch. If a person has taller rifling or a slower twist rate, he can maintain rotational control (center) better, longer in the face of fouling that is inevitable at some point and still launch the bullet well.

The faster the spin (twist) will stabilize a poor launch sooner, provided that it doesn't pass the point where the rifling height can counter the increased rotational force and hold bore center until muzzle exit.

When the bullet exists, the launch is never truly .... perfect. The wider the nose, the greater the force from air to divert the slug until the bullet comes back to sleep in what ever direction it is going now. (We lost control when it left the muzzle)

The higher the muzzle pressure upon exit, the wilder the kick out will be. Too great and we key hole.

Failure to understand that bullet fit is an all encompassing factor from the throat to muzzle exit will ultimately set a lower RPM, or velocity, or fouling limit, for that gun or individual than someone else.

[Blacksmith]

Bret4207
Alright Bret, I’ll bite lets say I take a chamber cast and slug the barrel and from those draw the cross section of my guns internals from bolt face to rifling with dimensions. Do I need other data? From all of the above how do I design or find a “good/ideal fitting boolit”?

Gohon
In Hatcher’s Notebook, I can’t lay my hands on mine at the moment so this is IIRC, there was a section about penetration of 30-06 FMJ. I believe he said that in some tests because they were too close the bullet wouldn’t penetrate as well because it had not had time to stabilize however at longer ranges after stabilization it would penetrate more, I think it was 32” of oak. I believe the picture showed the unstable bullet had veered off track inside the oak.

Blacksmith

[Catshooter]

Bret,

Hah! Yer a troublemakeryaknowthat!?! :) But in a goooood way . . . :) ; )

You're right though, the fit is what the fit is. I have a .38-55 Winchester 94. Grooves measure out at .3795 and my onlyest mould would drop WW boolits at .3790 after I lapped out as much as I dared. The load pushes that 285 grain boolit at 1750 fps.

So I though, what the hey? Let's try it. Shot an inch and a half tall by three inch wide group at 150 yards. Go figure? No leading either. That rifle is my silhouette piece and I haven't cleaned it this year. Little carbon, but no lead after about 600 rounds.

Sometimes you can over think/measure/worry it. Sometimes it pays to just shoot it and see what happens, despite how bad you know it'll be.

Cherokee,

Sorry bud, but water is too compressable. Newton didn't think so, but those days are gone.


Cat

[Pat I.]

Bass let's just agree to disagree about what bullet fit is so this doesn't turn into another fiasco. As for myself I'll describe it as fitting the bullet to the throat and none of the other things you talk about. If you want to describe fit as everything from ignition to arrival at target that's fine with me and I'll respect your opinion.

[243winxb]

Size, lube, alloy, velocity, pressure all play a big part. If one is not correct all will not be well. Bullets leave the muzzle on a right hand twist, nose up, pointing to the left. They stabilize(go to sleep) around 75 yards or so. The reason the carbine didnt lead with 2400 is because 2% tin was added with a great powder.

[Bret4207]

Bret4207
Alright Bret, I’ll bite lets say I take a chamber cast and slug the barrel and from those draw the cross section of my guns internals from bolt face to rifling with dimensions. Do I need other data? From all of the above how do I design or find a “good/ideal fitting boolit”?


Blacksmith

IMO it depends on if you're speaking to basic fit or ultimate fit. That's the tricky part of figuring out this definition. In basic terms with the measurements you have you have an idea of where to start. Say the throat measures .310 at the largest area. Then you'd know, if the measurement is accurate, that anything over .310 will be an interference fit. So you have the basic "fit". But that isn't all there is to it. If you're shooing a long, lean bore rider with a body of .310 but a nose of ..300- does it fit? If you're shooting a blunter FN boolit with a tapered body starting at .300 and quickly becoming .308 and then later a cylindrical .311- does it fit? If you have a .311 boolit that will chamber as long as there nose is off the leade .250- does that fit. If you what seems to be a perfect design, something like an old stalwart RCBS 30-180FN and it measures .311, but shoots wildly and leads with one load but not another- does it fit? What if sizing to .312 fixes it with that one "bad" load- does it fit now?

In short the ultimate fit depends on what the gun wants for that particular boolit, seating depth, powder/primer, alloy, lube...even temperature can make a difference.

If someone can come up with a better idea than admiting there's basic fit and ultimate fit I'm more than willing to listen.

[243winxb]

Defining boolit "fit" Its .001" over the slugged groove diameter. Manufactures of swaged lead bullets already have set diameters for correct bullet fit for 99% of firearms, mostly handguns. Its about .001" larger than there jacketed bullets. Most new casters would do well following Lyman's cast bullet instructions. We can all learn from the high tech info experts place here, but hard to tell one from the other.

[243winxb]

lets say I take a chamber cast and slug the barrel and from those draw the cross section of my guns internals from bolt face to rifling with dimensions. Do I need other data? From all of the above how do I design or find a “good/ideal fitting boolit”?
My thoughts- Sizing to fit the throat, why? To center the round in the chamber or to reduce gas cutting? It would seem .001" over groove would be OK here. Why? Size your brass using custom bushing dies. The bushing should only size 1/2 of the neck or only where the bullets bearing surface touches. So now we have brass fitting the chamber and the neck tension is just right. Now , what bullet diameter should we size to? The bushing die centers the neck/bullet in the chamber, so thats good. How about gas cutting on firing. Well, if the nose of the bullet seals the bore before the base of the bullet leaves the case, there will be no gas cutting. The bullet jump to the rifling , COL is less important as the round is now centered in the chamber. Without bushings the Lyman "M" die would seem to help center the rounds neck? What it all really comes down to is "Testing" Find what your firearm likes. My 30 calilbers like .310" to .311" but i have not tried bushings.

[44man]

I do not have any problem with .430" boolit shot from a .430" bore and .4324" throats. .432" might be better but I can't prove it.
Now as far as twist turning a boolit or making a bullet tumble, I believe it is bullet shape first.
Shooting water jugs with the .475 showed a perfect straight path through 14 jugs and it blew up the first 4. Looking back at jug alignment and shooting position showed a straight path.
If the boolit is coming out the side, I would say it is boolit deformation, expansion, etc.
Over spinning a bullet can make it tumble but does it start with bullet shape? However, I do not see this happening with a revolver. I have very fast twists with my BFR's and have never seen anything but straight paths through animals or anything else I shot, even 16" of oak.
Deformation of the boolit would be my guess.

[Pat I.]

My thoughts- Sizing to fit the throat, why?

Because with either rifles or handguns it's one of the most important things you can do to get accuracy. Just because you're running a .308 bore size it doesn't mean the throat's anywhere near that. Fill the throat up and the rest just got a lot easier.

[Bret4207]

Agree with Pat 100%. Sizing to the throat is a very basic rule with case. The closer to throat size the better the boolit is supported. Boolits under throat size often give poor results. The tricky part is defining the throat and how best to fit the boolit to it.

[runfiverun]

gohon i was talking about impact/penetration not flight.
i too have found a slip fit [diameter] to work very well in many guns not just revolvers.
i think fit is when your boolit shape basically mimics the throat area of your rifle.
a bore rider does this differently by filling the lands first then the bore, they work differently.

[montana_charlie]

As for myself I'll describe it as fitting the bullet to the throat and none of the other things you talk about.
This is the correct way to view it.

If you dig a hole and fill it with concrete, that hardened chunk will 'fit' the hole.
Pour a chamber cast and create a bullet which fills the void, and you have achieved 'fit'.

All of the other things...hardness, velocity, RPMs, etcetera...those are things used to make the design work the way you want it to.

The 'shape' determines 'fit'.

CM

[jonk]

The importance of throat fit... yes..... in an ideal world, each gun we shoot would have a throat that 'ended' in good rifling right where the ogive of our bullet was seated for easy chambering, but with the very tip of the nose already in the rifling, with a bullet of proper weight for the rifling twist and speed desired.

It isn't a perfect world though. I appreciate some aspects of the need for proper throat fit, but I'd rather have a bullet that fit the throat poorly but was well made and sitting over a good load and sized to just over groove diameter, than one that fit the throat well but had other issues.

For instance, my M95 Steyr carbine has a rather worn chamber and throat. It will (just) chamber a .338 bullet, but has .332" grooves. Do I want proper THROAT fit with that hugely oversized bullet? No, it deforms so much that accuracy is very poor. Or something. OTOH while no one will ever call it a tackdriver, it does acceptably well when sized to .334- a balance between oversized enough to engage the rifling, but not leaving a TON of space in the throat either.

It's such a crapshoot. For any individual gun, you have to determine:
1. What weight bullet it likes.
2. What profile bullet it likes.
3. What powder/primer/case combo it likes with said bullet. And how much of that powder.
4. Seating depth
5. Possibly lube type and amount

And I"m sure a ton more. Let's suppose we had a Lyman 311284- very nice bullet, great mold. Suppose we were sizing that bullet to .311 and shooting it in a 30-06 with sharp bore that slugs around .3095. Sounds good so far. Let's even suppose that we seat it .001 off the lands as a good start point.

Guess what? The same charge that works great in my 03A3 with similar seating depth, similar bore dimensions, similar everything, shoots poorly in my plain 03.

I'm convinced it's all based not on physics but on the mood of the little gremlin inhabiting each gun.

[montana_charlie]

The importance of throat fit... yes..... in an ideal world, each gun we shoot would have a throat that 'ended' in good rifling right where the ogive of our bullet was seated for easy chambering, but with the very tip of the nose already in the rifling, with a bullet of proper weight for the rifling twist and speed desired.
Your description is not that of a 'bore rider', and that is fine with me. I'm not in a love affair with bore riders.
But, your definition of 'fit' doesn't sound right to me.

I would prefer a nose shape that resembles the angle of the leade, so the contact point is long and firm. The nose should be well up in the leade if it is a spitzer, but that isn't a requirement for establishing solid contact, even if the angle is shallow.
One driving band (at least) would fill the diameter of the freebore, if one exists.
This provides two concentric contact points which align the bullet with the bore.
The overall length is proper for the twist of the rifling, and that part of the bullet not required to 'fill' the throat is seated within the case.


It isn't a perfect world though. I appreciate some aspects of the need for proper throat fit, but I'd rather have a bullet that fit the throat poorly but was well made and sitting over a good load and sized to just over groove diameter, than one that fit the throat well but had other issues.
If you settle for a bullet that is a poor fit, you will never realize the full potential of that rifle.

For instance, my M95 Steyr carbine has a rather worn chamber and throat. It will (just) chamber a .338 bullet, but has .332" grooves. Do I want proper THROAT fit with that hugely oversized bullet? No, it deforms so much that accuracy is very poor. Or something. OTOH while no one will ever call it a tackdriver, it does acceptably well when sized to .334- a balance between oversized enough to engage the rifling, but not leaving a TON of space in the throat either.
Here you are stating that you get 'acceptable' performance from a known 'poor fit', but you can only use 'or something' to say why a fully-fitted bullet doesn't work as well. Perhaps you aren't picking the right ones out of that box of 'other variables' we all get to dig around in.

Let's suppose we had a Lyman 311284- very nice bullet, great mold. Suppose we were sizing that bullet to .311 and shooting it in a 30-06 with sharp bore that slugs around .3095. Sounds good so far. Let's even suppose that we seat it .001 off the lands as a good start point.

Guess what? The same charge that works great in my 03A3 with similar seating depth, similar bore dimensions, similar everything, shoots poorly in my plain 03.
Guess what? You say that everything is similar between the two barrels, but you didn't say that two chamber casts are essentially identical. Maybe they would be...maybe not. But, you didn't say...

What you did say is that identical loads - with the same powder charge - perform differently in the two rifles.

If both chambers were cut with the same type of reamer, it's safe to assume that bullet fit is about equal between them. If true, choosing a different 'variable' (powder charge comes to mind) might be the deciding factor.

If the bullet doesn't actually fit either of them very well, then you're into that crapshoot you mentioned...and simply having better luck with the 03A3.

The way to avoid the crapshoot is to search for 'the established design' which was developed for your particular make/model/caliber...or to get a mould from a maker who will make adjustments to match the chamber cast you provide.

CM

[bearcove]

Aw man, you guys ruined it! I thought this was a simple relaxing hobby.

1 Make boolit
2 Load Boolit
3 Load gun
4 Kill can

[smilie=s:

[Bret4207]

This is the correct way to view it.

If you dig a hole and fill it with concrete, that hardened chunk will 'fit' the hole.
Pour a chamber cast and create a bullet which fills the void, and you have achieved 'fit'.

All of the other things...hardness, velocity, RPMs, etcetera...those are things used to make the design work the way you want it to.

The 'shape' determines 'fit'.

CM

The shape and dimensions determine basic fit, or lack of basic fit. But when you change alloy and it drops a different size the basic fit is changed. When you boot a boolit so hard the base obturates then you've altered the fit. I understand what you're saying, but I feel it's more complex than just limiting things to shape and dimensions. If we're going to be able to explain this to the noobs, we need to agree on standard terms.

Thoughts?

[Pat I.]

Just my $.02 but a bullet either fits or it doesn't. If you have a .310 ball seat throat and run a .3095 bullet it fits. If you run a .311 or .308 bullet it doesn't fit as well. If you boot a bullet hard enough to change it's physical dimensions that's a loading issue and nothing to do with fit. If your throat is cut off center or the muzzle is cut on an angle or has a worn crown that's a mechanical issue that needs to be addressed. If you push a bullet past it's strength that's an alloy and hardness issue that can be changed. Fit is fit and the rest is something else. The very best way to fit a bullet is to have a ball seat throat cut with a shallow taper and have a die made to either bump or taper the bullet to match. I don't have one but I think of the factory guns Weatherby would be the most cast bullet friendly rifles made because of their long freebore/ball seat.

[montana_charlie]

The shape and dimensions determine basic fit, or lack of basic fit.

Thoughts?
Try this, Bret.

Below are three pictures showing different 'shapes' and how they fit in a single chamber.
I don't have the software to create these, or to modify them very nicely, so disregard any roughness you see in the quality. Just consider what I point out.

Here is a basic bullet design which 'fits' the chamber.
Dimensions are suitable, and the nose has a slope that snuggles nicely into the angle of the leade.
Counting the 'long' contact at the nose, and the freebore-filling diameter of the first driving band, you have two positive points of support to align the bullet with the bore.

http://castboolits.gunloads.com/picture.php?albumid=88&pictureid=489



Here is the same bullet, but after being stretched into a bore rider and having the nose reduced to bore diameter.
You have contact up in the actual bore, and that driving band still provides the second contact point.

http://castboolits.gunloads.com/picture.php?albumid=88&pictureid=490

Finally...
This is the same bullet after butchering some length off of the original nose.
The nose diameter is too big for a bore rider, so it should have a slope that (at least) comes close to matching leade angle...but it doesn't. The leade is considerably more shallow, so if the nose was touching the leade, it wouldn't be a very firm touch.
But, because the design has such a short nose, it doesn't even reach into the leade to provide any kind of support.

http://castboolits.gunloads.com/picture.php?albumid=88&pictureid=1579

The 'diameters'' of this bullet are all still correct for the dimensions found within the chamber and bore, but as you can easily see, the SHAPE DOESN'T FIT.

Thoughts?

CM

[Blacksmith]

Montana Charlie
Thank you for the drawings. I can read drawings better than I can read reading so I see exactally what you are talking about.

Blacksmith

[303Guy]

Nice drawings, montana_charlie!

I have attempted to shape my boolits for my 303 Brit and 25/303 to fit bore, rifling leade angle, 'free-bore' and case neck.

http://i388.photobucket.com/albums/oo327/303Guy/BOOLITPROFILE-1.jpg
(A far less sophisticated drawing but hopefully it gives the idea).

The bore-ride section actually has a slight taper to suite the wear in the bore. The breach end bore is a little bigger than at the muzzle, which I consider to be perfect. The bore-ride nose will not enter the muzzle end. The boolit rear section is somewhat larger than the groove diameter, as is the 303 Brit throat. (It's a paper patched boolit, so has no lube grooves).

[45 2.1]

The 'diameters'' of this bullet are all still correct for the dimensions found within the chamber and bore, but as you can easily see, the SHAPE DOESN'T FIT.

Thoughts?

CM

Since you asked for thoughts, think about this. You are fitting two points, both of which are on the front end. What exactly are doing about the tail end, which has 0.006" or airspace (in the three pictures) around it in which to bend or get knocked out of line (an annealed case neck doesn't hold that in line either). You really need to fit the rear end also. The basic problem of little accuracy gets solved mostly when you fit the rear also. There are other factors involved that see little press here.

[montana_charlie]

Since you asked for thoughts, think about this. You are fitting two points, both of which are on the front end. What exactly are doing about the tail end, which has 0.006" or airspace (in the three pictures) around it in which to bend or get knocked out of line (an annealed case neck doesn't hold that in line either).
I guess the .006" you refer to is the difference between the diameters of the chamber and the case neck...right?
Remember, I don't have software capable of creating it, so this is not my drawing. I 'dinked around' with a diagram I had previously 'collected', in order to make a visual aid for fitting a bullet to a 'throat'.
So, I only modified that part of the picture.

But, since you ask, I agree that support of the rear end of the bullet is also important. My genuflection to that requirement is to only shoot fireformed brass, and my bullet's shank just fits in that ID.
But, I realize that using unsized brass isn't possible in all shooting situations.

CM

[45 2.1]

I guess the .006" you refer to is the difference between the diameters of the chamber and the case neck...right? Correct
Remember, I don't have software capable of creating it, so this is not my drawing. I 'dinked around' with a diagram I had previously 'collected', in order to make a visual aid for fitting a bullet to a 'throat'.
So, I only modified that part of the picture. My "thoughts" were about your statement "The 'diameters'' of this bullet are all still correct for the dimensions found within the chamber and bore". I don't agree that the diameters are correct if you want excellent accuracy.

But, since you ask, I agree that support of the rear end of the bullet is also important. My genuflection to that requirement is to only shoot fireformed brass, and my bullet's shank just fits in that ID.
But, I realize that using unsized brass isn't possible in all shooting situations. This is true, especially when loading accurate ammo for a semi-auto.

CM

[montana_charlie]

My "thoughts" were about your statement "The 'diameters'' of this bullet are all still correct for the dimensions found within the chamber and bore". I don't agree that the diameters are correct if you want excellent accuracy.
You are welcome to disagree based on whatever parameters you feel are important. However, the indicated groove diameter is .308" and the bullet is sized to .310".
The .310 bullet will actually swage down to .308 in the slight taper of the freebore, shown by the dimensional labeling of the diagram.
I believe we all (with few exceptions) will agree that a bullet 'two thousandths over groove' is an acceptable diameter for a given bore, in most instances.


But, I realize that using unsized brass isn't possible in all shooting situations. This is true, especially when loading accurate ammo for a semi-auto.
Considering the 'tolerances' required to allow a semi-auto to function reliably, it may be that a 'perfectly fitted' bullet would be a bad idea in that type of arm.
Actually, those same tolerances probably preclude the degree of accuracy which 'perfect fit' is meant to enhance.

In any case, action type is outside the scope of how a given bullet fits in a steel hole. If the thickness of the neck wall (in the picture) allows too much 'slop' for good accuracy, that is a brass preparation issue.

CM

[runfiverun]

it seems that if the third one had a bit less driving band so the boolit could be seated somewhat more forward,that would be an excellent fit even with a smaller diameter.
like groove diameter.
a bit more flat nose could help with this.

[Bret4207]

Charlie, Pat, etc- The issue I have is sort of addressed in the preceding posts- yes, shape as shown in Charlies drawings determines some parts of fit. But the drawings don't cover it all. I'm a big one for the +.002 groove type of "fitting". That's all well and good, right up until I run into the gun that wants things at groove diameter! So WHY does it want them there? Because that's what fits. So how do we describe that? How do we explain why that is what one gun wants for fit, but not another? Nothing here so far does. Yeah, sometimes its the load,. but sometimes it's not. If we have a seater that squashes the diameter down and creates a wasp waisted boolit we've altered the fit. If we "bump" a boolit in the sizer we've altered fit.

I realize I'm picking at minutiae here, but this has been in the back of my mind for sometime. I disagree that obturation is a loading issue IF that's what we're depending on to make the boolit fit. We may have some change in the boolit on ignition and not realize it and that's all part of the fit of the boolit. That's why I refer to basic fit (what Charlie describes) and ultimate fit. It's all part of the same effort to give the gun what it wants.

[montana_charlie]

That's all well and good, right up until I run into the gun that wants things at groove diameter!
When you run into that one, do you buy a custom mould for it...a mould that casts a bullet which 'fits' that chamber?
Do you have a custom mould for any of your rifles...made to the specifications obtained from a chamber cast?

When some standard practice like 'two thou' over' doesn't get the job done, the shooter starts making adjustments. He starts digging into the box of variables that we all have to confuse and amaze ourselves with.

Maybe it's a diameter change, or maybe it's just seating the bullet to a different depth. But finding the right variable(s) can make a poorly fitting bullet perform with reasonable accuracy. (And, there are some real loose definitions of 'reasonable' out there.)

But...(choke!)...what if the bullet' shape and size actually fit the chamber to begin with?

A bullet either fits or it doesn't...although there can be the case where a bullet almost fits. But 'almost' is like grenades...close might count, if it is 'close enough'.
When the fit is there, you have what you (Bret) like to call 'basic fit'. If the fit changes when fired - due to obduration, slump, or whatever - then you have your 'ultimate fit'.
If the bullet doesn't have to change much to assume that nook-and-cranny-filing configuration, the design can be expected to perform. This is also where that 'almost' fit can be useful...if it is 'close enough'.

But, the more modification a bullet goes through between the time it falls from the mould and the point where it is halfway up the bore, the more it's performance can be expected to degrade...assuming it was a fitted design to begin with.

A new shoe may wear out too fast to be worth it's cost, might be the wrong style for the kind of parties you go to, or could be too warm for summertime. But you know if it 'fits' when you stick your foot in it.

Unfortunately, it's not that easy to tell if a bullet fits a particular 'steel hole'. Thank goodness there are bullet designers and custom mould makers who can.

CM

[Pat I.]

Bret maybe I've just been lucky all along but I don't go for the "every gun's an individual" idea. If that was the case we'd all spend the rest of our lives trying to get them to shoot. There's things that work for every cartridge and it's pretty easy to find out what it is by asking for loads. They might not shoot as well in your gun as the next guys but they will shoot. Any difference is more of a bedding, barrel, crown, etc. ect, ect. issue than the fact that every gun using the same cartridge is so different that they require different alloys, temper, bullet, lube, yada, yada, yada.

Fitting a bullet to the throat is the foundation of getting them to shoot and it's something that's measurable, not voodoo. It might require some work but it is doable. If you're running something like a .308 that uses a ball seat throat make a throat slug and measure the ball seat, this minus a half thousandths or so will be your diameter. If you're running a bore ride find one or adjust your alloy so it's a slight interference fit with the lands and the bullet will be supported in two spots. This is fit. If you really want to go whole hog form some brass out of 30/06 and turn the necks so there's no more than .002 clearance with the chamber and you've got the best set up you can that doesn't require rethroating the rifle. An interference fit might be better yet

If you're shooting something like the 30/30 that has a steep taper from the end of the chamber neck to the bore you have to consider the cartridge neck itself the throat and use a diameter that'll give you a .001 or .002 clearance with a loaded round. Get fancy and clean up the neck and it'll be a good fit and straight with the bore. The same thing that applies to bore ride bullets with the .308 applies here. A simple test to see if you're on the right track with fit is to push a sized bullet into the throat with a dowel or something and shine a strong light down from the muzzle. If you see light around the bullet your fit is bad, if it's plugged up so no light get by the bullet you're heading in the right direction.

People have to get over the idea that you have to use the heaviest bullet available or size to .002 over groove diameter. I know a couple of guys that where shooting .314 throats in CBA competition with .313 or .314 bullets and they won nationals and set records. They were both using bumped 314299 bullets. If you have to shoot a lighter bullet because you have a short throat you'll be a lot better off doing that than trying to use a heavy bullet that's hanging down in the powder area or the nose is rattling around in the bore.

I personally think "Obturation" is a word that gets used a heck of a lot more than it should be and is used as an excuse for something else out of kilter. Maybe with 20-1 or 30-1 and black powder there's something to it but with the alloys and loads commonly used by smokeless gas check bullet shooters I consider it a non issue. And even if it did become an issue a simple alloy change or HTing will solve the problem.

[Bret4207]

Hmmm. Lately I've become a fan of the "measure a case mouth fired with a factory load and start there" method instead of slugging. It seems to work. So maybe I'm over thinking this or maybe I'd be better sticking with my basic and ultimate fit ideas.

Time to dwell on this...

[montana_charlie]

Hmmm. Lately I've become a fan of the "measure a case mouth fired with a factory load and start there" method instead of slugging.
Then, I guess I'm done...
CM

[1917]

This is a lot of info to digest. I'd rather be shooting something.

[Bret4207]

Then, I guess I'm done...
CM

Oh. don't give up yet Charlie. I just need to think of a better way to explain what I'm thinking.

[Pat I.]

This is a lot of info to digest. I'd rather be shooting something.

I agree it does seem like a lot to digest but in reality it's real simple. It just seems like a lot because you can't actually look at what we're talking about. That and the fact that people have a tendency to want to over complicate things and use big words to describe a hole and a taper.

[felix]

Best example I have seen showing the effect of blasting a boolit without chamber wall support. The "pressure" exhibited here is minuscule compared to the real thing. Also, think how tough a baseball is compared to a boolit. The baseball comes back to "round" fairly easily even though it is "soft". ... felix

[Pat I.]

But you forgot this part which is why I said black powder's a different story.

[303Guy]

Ahh, but .... what if the 'SLOWER POWDER' produces the same pressure as the 'FASTER POWDER'? i.e. The same peak pressure and the same mean pressure with a resultant 'same' velocity?

http://i388.photobucket.com/albums/oo327/303Guy/CHAPTER-6-PAGE-90-FIGURE-6C4.jpg

[runfiverun]

it does and can but it does it later in the curve.
the way a slower powder gives higher velocity is by sustaining the gas volumn longer
but with the same peak pressure.
it might even [does] sustain the higher [not highest] pressure longer. so the pressure and time it's applied is how velocity is produced.
so you won't have same reults with a faster vs slower powder.
you might get the same velocities and even the same peak pressure but not the same results on target.
yes you could use 15 grs of red dot in your 0-6 with a 150 gr bullet or use 60 grs of rl-19.
for a peak pressure of 60k.
but i bet the velocities are about 1,000 fps different. and gas volumn is why.

[felix]

Shows why tight neck measurements are critical for maximizing both accuracy and safety at the same time. ... felix

[montana_charlie]

Ahh, but .... what if the 'SLOWER POWDER' produces the same pressure as the 'FASTER POWDER'? i.e. The same peak pressure and the same mean pressure with a resultant 'same' velocity?

http://i388.photobucket.com/albums/oo327/303Guy/CHAPTER-6-PAGE-90-FIGURE-6C4.jpg
Your graph falls outside the parameters of the equation.

Note that the original illustrates two effects which are different because of only one variation...the 'quickness' of the charge.
The area encompassed under the two original curves is equal in both instances because the 'weight of charge' does not change...it is not a 'variable'.
The encompassed area is meant to denote the amount of energy generated by a given amount of fuel. The 'quickness' of that fuel causes the force to be developed at varying rates, but the total amount of force is a constant.

Your curve encompasses (roughly) twice the area of either of the other curves...meaning you used twich as much fuel to create the force it represents.

Therefore, your argument is invalid.

CM

[Pat I.]

The reason I posted the graph was to point out that smokeless powders are progressive and don't reach peak pressure at ignition like the baseball example. I don't think a tight neck (as long as it's not too tight) has anything to do with safety and is all about alignment.