What is "fit"?

[btroj]

Let's discuss what fit means to us. This will be a discussion about factory rifles, custom or BR jobs often have dimensions tight enough to change things.

Fit in a revolver to me is simple, I want a throat larger than bore diameter by a bit and a bullet that is snug in throat, at or even .001 over throat diameter. We are actually fitting a cylinder k the throat, so it makes things simple to me.

In a rifle things get a bit stranger.

Take the bullet I am using in a 375 right now. The bullet is seated long enough that e throat pushes the bullet farther into the case when the bolt is closed.the bullet is sized larger than bore diameter and gives no leading. It is NOT a bore rider so nose diameter isn't an issue.

What makes this bullet really "fit"?

Is the bullet diameter to bore dimension the critical factor?

Is it the nose of the bullet, actually a front band, being snug into the lands a good fit?

Would a bore rider with a nose large enough to lightly engrave the nose be better in the fit department?

We can also get into the difference between a bullet with a distinct front band as opposed to something with a nose that gently tapers into the full diameter body with no front band- more like the Ranch Dog style bullets.

I personally feel that a bullet with a snug bore ride nose or the taper like the Ranch Dog style give the best accuracy. I think thy center up in the bore the best, at least in a factory throat with a large neck dimension. I like to seat them so the bullet is snug into the lands as the bolt closes. I usually find that see bullets show land marks well into the nose, even io areas that don't show engraving on a non-fired but chambered bullet. This shows that the bullet is slugging up a bit upon firing, this gives a good dynamic fit under pressure.

How do the various parts of a bullet affect fit? I feel the nose shape and how it enters the throat is the most critical factor. Is is assuming a bullet that is sized to . 001 over bore diameter.

A loose fitting nose on a bore rider is going to give poor accuracy in many cases even though the body of the bullet may fit. Would a proper sized nose overcome a slightly under sized body? Do we need both? I think we need both.

We speak so often of bullet fit but what does that really mean? I think it goes beyond the compairison of bullet diameter to bore dimensions.

This doesn't even begin to take into account "dynamic fit" which is the fit of the bullet under pressure as it travels down the barrel.

I am certain I have left out soooo many areas that I will kick myself later. I just think we need to look at how "fit" fits into our vocabulary. What does it mean to you?

Brad

[geargnasher]

Let's discuss what fit means to us. This will be a discussion about factory rifles, custom or BR jobs often have dimensions tight enough to change things. Hmmm, I'd say it's the other way around, factory rifles sometimes, if not mostly, have dimensions LOOSE enough to change things away from the "ideal". You might also mentally underline the fact that "tight" and "important" go together here.

Fit in a revolver to me is simple, I want a throat larger than bore diameter by a bit and a bullet that is snug in throat, at or even .001 over throat diameter. Depending on lots of things like alloy, pressure curve, peak pressure, the particular details of the gun's internal shape, some revolvers prefer a slighly undersized boolit, where others seem to like a fuzz larger than throat diameter. We are actually fitting a cylinder k the throat, so it makes things simple to me. Sometimes throats aren't cylindrical to match, another monkey-wrench in the works.

In a rifle things get a bit stranger.

Take the bullet I am using in a 375 right now. The bullet is seated long enough that e throat pushes the bullet farther into the case when the bolt is closed.the bullet is sized larger than bore diameter and gives no leading. It is NOT a bore rider so nose diameter isn't an issue.

What makes this bullet really "fit"? Pressure and the shape of the barrel.

Is the bullet diameter to bore dimension the critical factor?

Is it the nose of the bullet, actually a front band, being snug into the lands a good fit? Not really.

Would a bore rider with a nose large enough to lightly engrave the nose be better in the fit department? Only on the nose part. That isn't the whole story, and by itself means little.

We can also get into the difference between a bullet with a distinct front band as opposed to something with a nose that gently tapers into the full diameter body with no front band- more like the Ranch Dog style bullets. This is all relative to the shape of the GUN. RD boolits fit a certain shape particular to Marlin leverguns for which they were designed, and some others, but are in fact the exact wrong shape to "fit" many others. Very important point here.

I personally feel that a bullet with a snug bore ride nose or the taper like the Ranch Dog style give the best accuracy. Both can be very accurate if they fit the gun. Notice there is something you didn't mention here. I think thy center up in the bore the best, at least in a factory throat with a large neck dimension. Large neck dimensions cause some other fit issues that cannot be fixed by centering the front end alone. You're getting warmer. I like to seat them so the bullet is snug into the lands as the bolt closes. That's one way to do it, but not the only way. I usually find that see bullets show land marks well into the nose, even io areas that don't show engraving on a non-fired but chambered bullet. This shows that the bullet is slugging up a bit upon firing, this gives a good dynamic fit under pressure. ....Or poor dynamic fit, depending on how good the STATIC fit was. Can you guarantee the "slump" will be the same every time if the static fit is sloppy?

How do the various parts of a bullet affect fit? I feel the nose shape and how it enters the throat is the most critical factor. Getting warmer! Tell us more about that, be specific. Is is assuming a bullet that is sized to . 001 over bore diameter. Every part of the boolit "affects fit". Every part is important. You still haven't said it.....

A loose fitting nose on a bore rider is going to give poor accuracy in many cases even though the body of the bullet may fit. May fit what? Obviously it needs to fit the groove dimension, what else? Would a proper sized nose overcome a slightly under sized body? Ummm, no. Do we need both? I think we need both. Still haven't said it....the most important part to accuracy.

We speak so often of bullet fit but what does that really mean? I think it goes beyond the compairison of bullet diameter to bore dimensions. Much more. This is where the majority of casters stop thinking about it, though, if they even go that far. Richard Lee's rifle boolit designs reflect this sort of two-dimensional thinking.

This doesn't even begin to take into account "dynamic fit" which is the fit of the bullet under pressure as it travels down the barrel. A point often missed as well. I'll give you a hint, it involves "fitting" the boolit statically so that "dynamics" is minimized as much as possible.

I am certain I have left out soooo many areas that I will kick myself later. I just think we need to look at how "fit" fits into our vocabulary. What does it mean to you?

Brad

Brad, you're full of good questions lately. Dividing the thinking into terms of both static and dynamic fit is a good way to conceptualize this.

Revolvers---dynamic fit is paramount, because the trip through the gun is an obturation nightmare for a boolit due to the high potential for leaks, loss of lube, and particularly uneven slump or setback. Statically, the boolit needs to be a fairly snug fit in the throat so it has the best chance of getting a straight start, which is paramount to accuracy in any gun. Having a snug chamber fit in a revolver doesn't always make an accuracy difference, but sometimes it does. Once the primer is struck, the boolit must break the crimp, make it's way out of the case and into the throat, then across the cylinder gap, feel out the forcing cone, pilot up in the bore (often while self-aligning the cylinder as it wiggles through, smack into the rifling and get yanked into a spin suddenly while it's already traveling at nearly top speed. After doing all that, it still must be able to obturate (seal) the bore, and make it through various restrictions with enough lube left to clear the muzzle. When it DOES clear the muzzle, it needs to have survived the trip with no uneven deformation, nose setback, or base distortion or it will go off on a tangent. Balancing the pressure to the alloy, the duration of the pressure curve, and properly fitting the boolit to the throats is only the beginning. The boolit must be shaped so that it can self-align as required without smearing off one side of the leading band (if any), swage to the rifling as required, and accelerate through and out of the gun while both maintaining obturation and resisting uneven deformation. Pretty tall order, it's a wonder it works at all. Shooting a lead boolit out of a revolver is like a triple-axel spin in ice ballet, if the balance of forces isn't perfect, you get a crash and burn.

Autopistols are pretty simple. The boolit has to be the right shape to fit the magazine, cycle through the gun, and pilot in the throat. The better the boolit fits the throat shape, and the sooner the boolit obturates the bore after beginning to move out of the case, the better. Little tricks like leaving just enough flare on the case to aid centering of at least the front of the cartridge (extractor pressure and loose case heads play hoc with alignment).

Now rifles. I'm sure you've been wondering what you were missing, and I'm going to ask more than tell here. Consider this: Perhaps you're firing a boolit which has a lower ultimate strength than the pressures developed during the transition into the bore. What do you think happens to the part(s) of the boolit which are unsupported statically when the powder lights? Think about that, think about what you said about "tight" in the beginning, the implications of "loose", consider that "straight" is concern Numero Uno through all of this, and also think of the mechanisms beyond the boolit itself that have a critical effect on "boolit fit" in a rifle, and you will have your answer.

Gear

[btroj]

Yes, the bullet expands under pressure to fill any areas it can. If things are right with the chamber and throat this happens in a uniform manner around the circumference of the bullet. Should it happen on one side more than another we make an unbalanced bullet.

To me this is where a poor fitting bore rider falls apart. Kick it in the rear with some pressure and the lead slumps some in the nose. Should the chamber allow the bullet to rest on the bottom of the bore it will tend to expand more on one side than the other, giving an unbalanced bullet.

If I understand what you are getting at it is this- the bullet needs a good initial static fit in order to have a successful dynamic fit. In other words if we have a static fit that can potentially allow variations or non-concentric dynamic fit then that is what will occur. An unbalanced bullet is the result leading to poor accuracy.

the bullet must not fit just the bore, it must fit snugly, and squarely, into the throat. The throat must have an angle that is capable of letting the bullet enter it smoothly and without extreme deformation. The bullet must fill the throat well enough to prevent excessive slumping of the bullet to fill the empty space. Case neck dimensions should be such that the gas check is at or above the bottom of the neck while the bullet is resting in the correct portion of the throat. This can be a bit tricky to find.

My entire purpose for posting this is to not only expand my own understanding of "fit" but to get people to understand that "fit" is a multifaceted thing. Fit is far more than bullet diameter. Bullet design, case design, and throat/leade dimensions need to be in harmony. A bullet that works well in a 30-06 may not do as well in a 308 with a shorter neck. The throat/leade dimensions we have limited control over.

I am beginning to realize that an impact slug of the critical throat area may well be more I portent than a barrel slug. I can easily guess what bullet diameter will work and know when I am right due to a lack of leading. The throat is far harder to see, an impact slug can let us know what we are working with and what we need to account for.

In short, good static fit can lead to good dynamic fit. Poor static fit may allow good dynamic fit but that dynamic fit may also unbalance the bullet. In the end we need a good static fit to allow a concentric, smooth dynamic fit. That will allow the bullet to remain balanced and give good accuracy potential.

Of course this all ignores the peak pressure and pressure curve shape due to burn
rate.

Brad

[geargnasher]

You got it, man!

When you kick a boolit in the pants hard enough, it will slump and deform anywhere it can. The inertia of the nose can lead to slumping, the rear can get shoved off to one side before it even enters the throat, the middle part can wad up or bulge to the side anywhere it's allowed to by the throat and leade, the whole thing can get a crooked start due to imperfections in how the case fits the chamber, the base can become unsquare resulting in yaw at muzzle exit, all sorts of bad stuff. We correct this by supporting the boolit everywhere we can statically, so it can only go straight and true with minimal damage as it swages into the confines of the barrel.

Copper jacketed bullets will "self align", or basically rattle around in the leade until they find bore center, get kicked straight, and go on their merry way. This is a function of the hardness and lubricity of the jacket itself: It can take the beating without deforming so much as to affect accuracy drastically in most rifles or handguns. Now to get the best accuracy from them, fit becomes once again paramount, but the corrections being made are on a micro-scale because the relative damage leading to relatively small inaccuracies is less drastic compared to cast boolits. Making a one MOA rifle into a .300" or even less rifle as opposed to taking a 4" group down to 3/4"

Obviously, as pressures and velocities are increased, the damage and the effects of that damage outside of the barrel become greatly magnified. Notice how easy it is to get air-cooled wheel weight alloy to shoot well out of a small-bore rifle at 15-1700 fps with no particular attention payed to the fit issues you've named. I believe this results in some misconceptions about shooting cast boolits, mainly because the factors affecting cast boolit accuracy aren't well understood by many of those shooting them. The rules of copper-jacketed don't apply in the same way, or actually they do, but it is far less obvious. Bring that up to 2K fps and things get a bit more tough. Go up above that and it's progressively more difficult, even with harder/tougher boolits. There are a variety of tricks, many involving fit, many involving other things, that will enable one to continue to squeeze the most repeatability from a particular platform as the performance level is increased.

Gear

[mpmarty]

Fit for me is anything I can close the bolt on, the bigger the better. I try to keep the nose a snug fit in the bore also.

[runfiverun]

Let's discuss what fit means to us. This will be a discussion about factory rifles, custom or BR jobs often have dimensions tight enough to change things.

How do the various parts of a bullet affect fit? I feel the nose shape and how it enters the throat is the most critical factor. Is is assuming a bullet that is sized to . 001 over bore diameter.

A loose fitting nose on a bore rider is going to give poor accuracy in many cases even though the body of the bullet may fit. Would a proper sized nose overcome a slightly under sized body? Do we need both? I think we need both.

Brad

i'll touch on this:
the "nose" must fit, does it have to engage the rifling?
nope.
it does have to mimic the throat shape quite closly,if part of the nose engages the rifling that's fine [even better actually].
on the bore rider:
the nose must engage the rifling slightly engraving of about .0015 [depth] is correct [this generally is not too much to overcome the neck tension and remain behind if the round is unchambered]
the nose must be concentric and engaging all of the lands equally.

the body of the boolit does not have to be anything oversized it must match the bbl.
however the bbl must be smooth and straight and the same diameter from front to back.
i use a 224 diameter boolit in my 223 rem rifle.
but a 225 diameter boolit in my 22-250.
same boolit different sizing dies.. same velocity.
and a 358 diameter boolit in my 358 win.
this rifle uses the saeco #248 the rear quarter of the nose on both the 224 and 358 lightly engage the rifling the rest of the nose is mostly engraved at firing.
the body of the boolit glides into the groove diameter,as gently as possible.
all three of these rifles shoot under an inch at 100 yds,and all three are running at jaxketed velocity's the 22's over 2700 and the 358 is in the +2300 range.
what do these rifles have in common the boolits are very similarly shaped and they are not bore riders.

you missed the part about nose/body relationship percentages.
the further to the body side you can go the harder you can generally push them.
however some rifles will not allow this so you must use a bore riding nose to keep the weight up this is when the nose engraving really comes into play, it will pick a case up off the floor of the chamber just enough to center everything.

if:
the nose is strong enough.
the nose is centered in the rifling as it is being chambered.
the case neck has a good enough grip to not allow any wobble.
the alloy will withstand holding everything up and not try and set-back [slump] when fired.
the last one splains why they generally will not handle higher velocitys

it's like a dart tip holding up the darts weight but the screw on tip is loose.

[.22-10-45]

Hello, everyone..good posts! Now there is another aspect of "fit"...the cartridge case. I am refering to length of case to chamber length. I have measured quite a few chambers & most are alot longer than any published trim-to -length suggestions in loading manuals...some alot longer..the .30 WCF is one of the worst. I have a custom .22 Hornet..close chambered by Mr. Ed Shillen himself..that has a chamber .03" longer than any commercial brass. When fired with cast, I sometimes find a thin black ring of lube .03" wide sticking to case mouth. I have had no leading problems with this gap when firing either plain-base or gas-check bullets..even as soft as 20-1 alloy.
However, in a black powder gun..I believe that soft bullet combined with the sudden kick of the black could obturate bullet into gap.
Has anyone took steps to fill this excess space?..Either by making cases from longer ones..or stretching originals?

[geargnasher]

.......you missed the part about nose/body relationship percentages.
the further to the body side you can go the harder you can generally push them.
however some rifles will not allow this so you must use a bore riding nose to keep the weight up this is when the nose engraving really comes into play, it will pick a case up off the floor of the chamber just enough to center everything.

if:
the nose is strong enough.
the nose is centered in the rifling as it is being chambered.
the case neck has a good enough grip to not allow any wobble.
the alloy will withstand holding everything up and not try and set-back [slump] when fired.
the last one splains why they generally will not handle higher velocitys

it's like a dart tip holding up the darts weight but the screw on tip is loose.

Bingo.

This is exactly what is happening when you: FL size the case in factory dies and stick a Lee two-diameter, bore-riding boolit in the case so that it pokes into the rifling as far as possible, then kick it in the pants with a load of 4198. If the boolit doesn't get bent when chambering the cartridge, it usually gets bent when fired and the case swells to fit the chamber, which you can bet doesn't happen evenly. At 1600 fps, and using the PSI/boolit strength formula as a guide, they can shoot surpisingly well because the boolit is able to resist permanent deformation while engraving. I think this describes the mechanics of a lot of the cast rifle boolits shot today. But start pushing the velocity up and all that slop really starts to take a toll on group size. There's nothing wrong with shooting mild, accurate cast boolit loads this way, but if you want "more" out of them you're going to need a better understanding of the limiting factors and the tooling changes needed to overcome them, especially when peak pressures are appoaching four times the alloy strength. Managing pressure curves and barrel harmonics becomes critical too, but that's a tale for another thread.

Gear

[btroj]

I agree that a bore ride nose must lightly engrave the rifling. I don't want it so tight thatit is hard to close the bolt or pulls bullets from loaded rounds when ejecting the round. A sloppy more fit lets the bullet to where ever it wants, a snug fit keeps it in the middle just long enough for the rest of the bullet to expand into the rifling.

So, ifi want a mould made by Accurate for a bullet with some bore ride is an impact slug the best way to get a good fit? It needs to be pretty close. I also understand that I can adjust the size of the nose .001 or so just be changing the alloy. I am looking at a 270 gr 375 bullet.

I have always had good luck with the RCBS 165 SIL bullet. Mine casts a large enough nose that it lightly engraves on the nose in about every rifle I have shot it in. I tend to seat it long enough that the front band is up against the end or the rifling if possible. I do like that the gas check is always going to be in the case neck. Maybe I have just been luck enough to have rifles with good chamber and barrel dimensions?

[pdawg_shooter]

Or...you can support the entire bullet with a couple wraps of paper and eliminate about 95% of your problems.

[Eutectic]

Maybe I have just been luck enough to have rifles with good chamber and barrel dimensions?

Lots of good info here Brad. I'll add a couple points that happen.... and can get you in trouble.

First, what is referred to as an "impact slug" is a good idea to see what you can see in the neck throat area.

Second, neck sizing.... I NEVER NECK SIZE! Why? You have no guarantee the neck will stay concentric with the body. I use a form of 'neck' sizing however by sizing the case body slightly (.001") along with the neck... This still may not conduce perfect alignment.... More with an example in a moment.

Not mentioned yet is neck length. Not on your case but in your chamber! I was quite into Contender pistols some 20 years ago. I bought a 14" .30-30 barrel. Try as I might I could not get a better group than 1 1/2" at 100 yards. (This was with "J" words) Maybe T/C figured '94 Carbine accuracy was sufficient? I could see by eye the lands abruptly traversed into the bore.... I could have throated it better with a countersink! I had not slugged the area at that time. I had a .30 caliber throating reamer. So I throated the barrel 3/16" deeper. Funny, it seemed like I could seat a bullet out about 5/16" farther? At any rate, with this 3/16" freebore area and bullets seated out this barrel would shoot!!!! Like 1/2" at 100 yards shoot! Again with "J" words. So I decide to use cast boolits. NOTHING IN MY BAG OF TRICKS WORKED. IT SHOT HORRIBLE! Like 6" groups at 100 yards horrible. Then I slugged the neck/throat area. I don't recommend waiting this long. Well, guess what? Some idiot at or for T/C designed a chambering reamer with a long neck... How long? About .100" over max case length! The moral of this story is check neck length chamberwise. A lot of damage can happen if our boolit jumps across this huge canyon that a too long neck area creates!

I use a Marlin Model 94 .25-20 a lot for lube testing and have said why on the "Extreme" thread. It shows you can't see everything with an impact slug either.
This Marlin chamber has some eccentricity at the shoulder lower neck area (maybe a couple thousandths) By the throat it appears gone and at the beginning of the chamber it seems OK too. Sorry for the vague description.... I would have to pull the barrel to accurately diagnose the mistake. Fired brass doesn't show it. IF you size the brass for a nice snug fit in this gun it will group 1 1/2" at best on my 70 yard target. Now I size brass in two different dies. The first die sizes the web area about .001" under a fired case and sets the shoulder back .004". The second die sizes the upper body a couple thousandths under a fired case and sizes the neck concentric with the whole mess. Now with a nose similar to the Ranch Dog boolit it groups 3/4" and usually better. By the way Brad, That nose shape is a good one in a short 'ball' throat like the early Winchester leverguns (and Marlin) have.

Lots can be in this 'fit' thing!

Eutectic

[geargnasher]

I don't neck size either, except for one caliber for which I use the Lee Collet die to 'bump' the case neck down a half-thousanth for paper-patching. What I do is reshape full-length sizing dies to fit my particular application. This means honing the neck and body areas to size only enough for reliable chambering (meaning only enough contact with the body of the case to keep it firmly aligned in the die, enough contact with the shoulder to "bump" it back a tiny bit, and making the neck portion not only size the minimum amount for boolit tension, but shaping the neck portionn so it leaves the base of the neck essentially unsized for an even more certain alignment. When the dies are worked over in such a manner, I can FL size based on the length of the shoulder (die adjustment) every case every time and not risk ruining the concentricity of the neck or case head. Every piece of equipment that touches the brass or boolit during the loading process must be modified according to the particular dimensions of the rifle, including turner pilots, seating die "necks", expander spuds, etc.

Another important thing that has been mentioned on this forum several times before is keeping the case centered during the initial fireform of the case to the chamber. This is important to case rim-to-body concentricity in rimless, bottleneck rifle cartridges, particularly in rifles with Mauser-type extractors.

Gear

[felix]

Accuracy is at least doubled when the case is prepared such that it does not exist dynamically. The objective is to make uniform acceleration curves. This thought process was just starting when the warehouse was closed, but enough experiments with turned cases with no expansion allowed were used just in time. No group fired was in the ones, and half below half that. ... felix

[geargnasher]

The objective here I think could be summed up as attempting to fit the boolit and cartridge to the gun so as to come as close as possible to a precision breech-seating condition with fixed ammunition by utilizing the primer and the initial powder burn as the breech-seating mechanism in such a way as to achieve the same benefits.

Gear

[felix]

Amen, Ian! ... felix

[btroj]

The key is how to make a factory chambered rifle work like that.

I mentioned originally I was speaking of a well made BR gun, they would be easier to set up because things are going to be tight, concentric, and true. The factory gun may not be.

At this point I am more concerned with getting the bullet into the rifling straight and in good shape more than I am how well the case fits the chamber. Not denying the case fit is important, I just feel the lead bullet is a weak link in ways due to the plastic nature of lead under pressure.

I can easily see where some chamber and throat designs/cuts are not going to be cast friendly. I have a Marlin 357 with such a throat, drove me nuts finding a bullet it would shoot well.

I need to make an impact slug of a the throat ons few rides, I think it would be a very worthwhile excercise.

[JeffinNZ]

It's a British term for an attractive person. EG: "That bird is well fit". LOL

[1Shirt]

It is a medical term for what you wife has when she learns that you have bought another gun that you can't afford!
1Shirt!

[bruce drake]

Also a legal term if you repeat it!

[geargnasher]

The key is how to make a factory chambered rifle work like that. Abosolutely! This is where we have to think outside the box and come up with methods to compensate for the inherent accuracy problems of "production"-type guns that must be able to safely fire a variety of different factory ammunition. Many military rifles are even more of a challenge. One person here is an expert at making some particularly "disadvantaged" military rifles shoot very well. There are some things, like damaged muzzle crowns, incorrectly bored/crooked chambers, etc. that will be limitations unto themselves, but a lot of other things can be "worked around".

I mentioned originally I was speaking of a well made BR gun, they would be easier to set up because things are going to be tight, concentric, and true. The factory gun may not be. That's part of what makes this so interesting.

At this point I am more concerned with getting the bullet into the rifling straight and in good shape more than I am how well the case fits the chamber. Those two really do go hand-in-hand. How can you start a boolit straight if the case is laying on the bottom of the chamber? If you have a scenario like Run outlined with the dart analogy, you are imposing some early limits on what can be done. Not denying the case fit is important, I just feel the lead bullet is a weak link in ways due to the plastic nature of lead under pressure. The boolit IS the weak link, that's why everything around it must be robust.

I can easily see where some chamber and throat designs/cuts are not going to be cast friendly. I have a Marlin 357 with such a throat, drove me nuts finding a bullet it would shoot well. Some are better than others for sure. I have an NEF .45 Colt that has an oversized chamber, about 1/4" of almost chamber-sized freebore, and an abrupt leade going into shallow rifling. I finally made it shoot, but had to essentially turn it into a .45/70, or more accurately, a wildcat based on the .45 Colt case.

I need to make an impact slug of a the throat ons few rides, I think it would be a very worthwhile excercise. Always a good place to start. Much of the time a pound cast makes a barrel slug irrelevant.

I'm learning that there really are two parts to making a production rifle shoot. First, you have to learn why it doesn't shoot straight. Second, you have to figure out how you're going to fix those things. It isn't always practical or possible to reform cases from another caliber for thicker necks, so that entails other "fixes". If you're like me, trying to assemble a fund of general knowlege about what it takes to achieve accuracy and what doesn't matter so much, it can be quite a challenge. The problems plaguing one rifle may well be totally different in another. Then you have to figure out all the other details involved with loading (beyond fit) which affect the way a particular gun shoots, and how to manipulate those things to your advantage.

So what do we do to solve fitment problems? I've mentioned some particulars about brass, and Runfiverun mentioned some other things about the boolit and the throat. Those are the two areas I look at first. Hopefully people will ask some questions and some specific things on how to fit these will be explained as the thread progresses.

Gear

[btroj]

So, does a bullet with a tapered ogive from nose to body diameter help? My thinking is that is it snugs up against the throat, which is hopefully centered, it will help hold the case and bullet centered upon firing.

Interestingy, this is a topic discuss in McPheraon's "Accurizing the Factory Rifle" regarding mods to the extractor of Marlin rifles. H uses the extractor to ensure the round is held in the chamber in a consistant manner.

Tis discussion is what I was looking for, it shows that fit is far more than bullets of a proper diameter.

[geargnasher]

This is where the pound cast comes into play. Observe the shape of the leade, how much freebore there is, the diameter of the freebore, and try to match a boolit to that as closely as possible. Some leades such as are common on Marlin leverguns are curved toward the bore centerline, like a torpedo's nose. Many other rifles have leades that are curved away from the bore centerline, like the bell of a trumpet. If the boolit's nose doesn't match this curve, it will bear in only one place and can pivot around on that point like a gimble.

If the boolit is only supported at the tip of the nose and the first driving band, the static fit may be fine, but when the pressure hits it it can buckle in the middle where it isn't touching anything and get bent slightly, or the nose can be pushed more into one side of the bore than the other and end up not being concentric with the bore. If the boolit's nose taper and curve match the throat/leade and the boolit is seated relatively close to contact (not necessarily touching!) AND the portion of the boolit NOT chambered into the throat/leade is held concentric to the bore at all times by the case, then the boolit has no choice but to go straight into the bore.

What has worked really well for me is to get as much contact surface area as possible between boolit and throat/leade, and to fill the throat completely.

Gear

[geargnasher]

http://castboolits.gunloads.com/imagehosting/thum_89095067bc7bf1908.jpg (http://castboolits.gunloads.com/vbimghost.php?do=displayimg&imgid=6865)


Here's a rough example of what I'm talking about, using some examples of some very challenging production rifle chambers with out-of-whack dimensions. Starting on the left, a boolit designed to fit the exact curvature of the throat and leade. It has shot some half-inch, five-shot groups at 100 yards recently at a shade over 2K fps, but not in the too-short, thin-necked case I show here. I intend to push these a bit faster with a little tougher alloy. The paper-patched "dummy" is also a custom-designed boolit, and has been chambered in the rifle intended for it, note the carbon smudges on the patch and how it fits the throat. This one shoots a little under MOA at around 2650fps. In the middle is a the 150-grain .270 RCBS boolit and a pound slug. I'm still fighting with this one a bit with case fit and alloy, so far getting it down to 2" groups at 100 yards from 6" groups. Last is an actual example from my standard hunting load that has shot numerous 3-shot 100-yard groups below a half inch from my MicroGroove 336. Note the huge gap at the end of the case neck on the pound cast, that was a challenge until I found the right boolit and loading technique. I have a pile of others, but that'll do for now.

A complete study of this would be the loaded round, pound cast, a sooted cartridge that had been chambered, the boolit before and after firing. I'm going to build an oiled sawdust boolit trap eventually to do this.

Gear

[runfiverun]

the real big issue we have is gravity.
we just measured everything within a half thousandth.
the fitted as well as possible within the ring.
then it all sits on bottom of the chamber and you end up with zero clearance on the bottom and all the clearance at the top with varying degrees in between.

you have to have at least two rings of the case suspending all of this with the boolit centered into the bbl.
quite often the two contact points are at the front and rear [boolit and bolt]
cheating with a donut on the neck,or leaving some case flair at the mouth will help,as will a partial sizing of the case body.
but that still falls short.

you can go with the flippty flop lee method hoping everything moves around and eventually centers itself.
or you can start on the long/big side and whittle things down [and the tools up] till things just squeeze in.
i view every rifle as a wildcat,and try to work with each one accordingly.
see lines below......

[geargnasher]

Pretty much everything, even the chamber neck, decreases in size toward the front end with bottlenecked rifle cartridges. I try to use that to my advantage when fitting case and boolit.

Gear

[runfiverun]

lets hear what everybody else has.....
this has had over 500 views, someone [s] has to have some opinions and/or questions,suggestions sumthin..

[btroj]

So a bit of extra flair on the case mouth that helps center the bullet isn't a bad thing? Never thought of that but it makes sense.

I have a few things to try.

[runfiverun]

i have adjusted enough flair to scrape when seating the boolit in a normal die.
sometimes that just isn't enough to align everything either.

[Eutectic]

The flare can produce excellent static alignment but is no guarantee of dynamic success. Seating the boolit tight into the throat area is better, especially with short boolits and the earlier 'ball' type throats. (A throat for a 1-3 R nose like a .38-40 boolit for example).

There has been much good advice given on case fit. But a perfect case fit in an imperfect chamber equals imperfect results. Chamber alignment to bore is hard to check but it can be done. One starts to realize why Pope and others did some of the things they did over a century ago. Muzzle loading the boolit down to an exact depth and chambering a case of powder is a very major inconvenience to a modern rifle design alright..... BUT.... It ELIMINATES any chamber misalignment problems, any cartridge case issues, it pre-conditions the bore every single shot, and VERY Likely would also eliminate the 'cold start' that some of us fight with diligence!

A correctly done deep throat is a good idea too. I have a .375 Winchester singleshot that I have throated to seat a loaded round with the Lyman boolit #375449 only 1/16" into the case. (a la Pope) Paper patched, it will shoot minute of angle. It does almost as well with lube grooves. This at just under 2000fps with 1 in 30 boolits. Most of the boolit is encapsulated into the throat (free-bore) in perfect alignment with the bore, and any dynamic distortion is longitudinal only. Circumferential alignment held in check.

I'll repeat.... There is a lot to 'fit'!

Eutectic

[btroj]

That is what I am looking to do Eutectic. I want to have a large enough bell on the case mouth to center the case mouth. I will also seat the bullet hard into the lands, that means the nose is centered pretty well. The case body, and neck, will be minimally sized to let it have the max amount of centering already done before ignition.

Sort of what I mentioned I don't know what can be done wi a factory chamber in 375 H&H.

[geargnasher]

"The flare can produce excellent static alignment but is no guarantee of dynamic success."

Brad, what Eutectic is pointing out here is it doesn't matter how centered things are statically if they don't remain so during firing. Essentially, flaring the case mouth only helps get the round chambered without bending the boolit in the instance of excessive chamber neck clearance. It will not compensate for that clearance during firing, though, unless you get really sneaky with what you use to push the boolit. Note that if you have a lot of clearance when loaded, it does not after firing. At what point in the firing cycle do you think the case neck gets that big, and what do you suppose happens to the back end of the boolit with that much room around it?

Gear

[btroj]

So if you have a large amount of case neck clearance you are screwed?

Tis rifle has shown a tendency towards good groups. Makes me beleive that at least this rifle will respond a bit to some tweaking.

I can see where a rifle with excessive neck clearance is no likely to release a bullet in good condition into the bore. In those cases what I am hearing that we can either shoot low velocity, and pressure, loads, or go to slow burn rate powders to reduce the tendency to bump up the base too much.

In the NE much of the end results are based pon the rifle and the chamber. A wel
Cut chamber with good dimensions will work easily, one cut out of round or overly large will give you fits and possibly ever give good results. Makes sense to me, a good rifle is a good shooter. Garbage is garbage.

[Eutectic]

So if you have a large amount of case neck clearance you are screwed?

Tis rifle has shown a tendency towards good groups. Makes me beleive that at least this rifle will respond a bit to some tweaking.



Brad,

If "this rifle" is the .375 H&H then you have an inherently accurate cartridge. If it's in a bolt gun and you want to feed from the magazine your options will be more limited.
The case is long with a lot of taper; the neck pretty short for the caliber.

When I look at these facts my thoughts go to very tight neck clearances first. This is probably impossible by normal means, but not impossible. My next choice would be to lengthen the throat with freebore. The more percentage of our boolit's bearing surface that is held in perfect alignment; then the higher percentage it has of shooting accurately.... I would encapsulate most of the boolit in a single shot rifle where overall length wasn't critical. This has worked for me in the smaller .375 Win. For the box magazine the rules change. A long bore ride section (which I'm not crazy about) and a minimum seated into the neck would be my next thought. This option would require a special designed mold.

We really need that concentric 'guide bushing' (the neck) to start our boolit correctly in most normal cases. Wouldn't it be nice if Starline made most rifle brass with overly thick necks with a BIG warning on the box or bag: "Caution.... The necks on this special dimensioned brass are overly thick. Neck turning for your chamber is absolutely required!"

Eutectic

[btroj]

Thick neck brass would be nice.

I am pretty happy with the results I am getting now, I do think the bullet is limiting my potential. The bullet I am using has o bore ride section and a long body. This means much is in the case neck. The bullet is snug against the throat but is held only by a front bad shoved into the throat, not ideal by any means.

I am leaning towards a mould with some bore ride section and a shorter, maybe neck length, body section. A snug fitting more in the throat should give much better fit into the throat at ignition because more bullet length will be in contact with the bore.

I need to make an impact slug and see how the free bore is, if any.

I do wonder of part of the reason I get good results from my RCBS 165 SIL bullet in 30 cal is because of the large nose that is a good firing the bores I shoot it in. That coupled with a short body section means that pretty much the entire bullet is supported by something at ignition. I have shot it in a couple guns with good result that make me think the design "works" well in guns with a snug enough throat and a decent chamber. One of these rifles is a bolt action 300 savage which can be tough with cast due to the very short neck.

Much to think about now.

[runfiverun]

the silhouette boolits have a reputation for accuracy they however are limited by velocity.
you have waaaay too much actually unsupported boolit there for velocity.
what's good for alignment isn't always good for everything else.
it's a trade off, remember there is no free anything in the gun worlld.
you gain alignment and low end accuracy.
you lose hunting efficiency,and high speed ability.
having a very short nose section that bore rides and a larger area that goes across the rifling to bore diameter and more bore filling contact is going to be a better bet.
remember your pictures of the 449 boolit?
a bore riding boolit would have not had any accuracy because it would not have been able to get the skidding under control like your longer groove filling boolit was able to.

[btroj]

What I don't like about the bullet I am using now is that sharp front band. I would prefer to keep the desinpgn as it but with the nose tapering into body diameter. I ink that would allow the bullet to better align with the throat while keeping the longer body.

For bore riders, ow much is too much for higher velocity? Is a 25 percent nose ok? 50 percent? The RCBS SIL bullets have a very long nose. How much more body is needed to give better high velocity results? I am thinking closer to 50/50 like the 311291 or a 311284 type bullet.

In this particular rifle I am looking for good accuracy in the 1600 to 2100 fps range.

Look at Accurate moulds designs 38 270b and 38 275 h for compairison.

[runfiverun]

in comparison of two similar but different nose/body designs.
i also have the rcbs 30-150 fn it is a duplicate in every way but it has one more drive band and less nose more a 60 body/40 nose instead of the more like 70 nose 30 body of the rcbs silh boolit.
i can always push the rcbs faster i have taken it to about 2400fps and maintained accuracy.
the silhouette will peter out at about 2,000 fps even when cast of a harder alloy.
it doesn't just open the groups at that point it starts flinging them to well over double it's potential.
i have shot several groups 1/2" and smaller with the silhouette boolit.
i am more in the 1" group size with the rcbs 150 fn but can maintain those groups up into the higher velocity.
most of my high velocity accuracy boolits have maybe 10-15% of a bore ride section,and about 10% of the nose that isn't.
the rest of the boolit lies in between the bore/groove maybe another 15% and the rest is full groove diameter.
so you have a very biased to supported boolit,and even if there is some nose slump there it is just filling up the groove a little more,and not going wherever in the bore to lands.

[popper]

leaving the flare on AP case does help center and seal. Bump should occur in the case to prevent ths start of gas cutting and expand the case. Additionally, out of round as dropped or sized would cause alignment problems. Nose and bore fit must account for tolerances in auto loaders.