Good afternoon Gentlemen-

A question for the "Experts"...

I am a student of CB's and shooting of the same and would like to ask a question regarding the best bullet diameter to use for accuracy. I am well aware that the current school of thought seems to favor a bullet that fits the "throat" vs. bullets that would fit the groove dia. for a rifle.

However, reading over the works of Phil Sharpe, Elmer Keith, et al., they clearly state that bullets of exact groove diameter or not over .001" over groove diameter are to be prefered. Bullets larger than this, they state, result in the bullet developing developing "fins" around the trailing edge. They claim these fins would catch the escaping powder gases as the bullet exited the bore and thereby upset(tip) it at that critical moment when it exits the muzzle and imparts yaw. They appear to have done quite a bit of accuracy testing and recovered bullets bore out their claim re: the fins.

So... What gives? New thinking vs. old - which is correct and on what basis? Has there been a change in the way rifles are throated that would account for the change in thinking or is something else at work here? One thing for sure - the current state of accuracy for CB's today is considerably better than back when - but so too it would seem are guns used to play these games. Your insights and especially sources of data to support your ideas would be greatly appreciated.

Best regards-

Sky C.
Longmont, CO

........I would like to suggest that fitting the throat or +0.001" over the groove has a bit to do with just how large the throat is. I'd think that something with an overlarge throat might just cause such a problem.

In addition to the throat, there is land diameter and land width, or bore form to consider. But the throat's the question, eh? The 'fitting the throat' idea is a point of further boolit alignment. If the slug should fit the throat to .001", or .0005" all the way around, then no major mis-alignment can take place. This assumes the entire cartridge shares the same centerline axis. We'll go with that thought.

For an example of some oddities in dimensions, I'll use my M95 Steyr. It has a .316" land diameter and a .331" groove. That's close to grooves .008" deep. Further, the rifle WILL swallow a loaded round with a .335" bullet. I know this because the first push through die I made for it ended up a trifle bulgy :-).

Since I'd spent so much time making it, I used it for awile. I never did get any accuracy worth speaking off. Bullets hit point on but the groups were large. Of course possible problems with bedding or bbl tension, not to leave out a pretty poor trigger may contribute.

Yet I can't help but think that possible 'over' engraving may have played a part. The barrel has lands almost as wide as the grooves. I was squirting a bullet that was .019" LARGER than the lands, and .004" larger than the grooves into the barrel. Almost 50% of the barrel surface engraves and as you mentioned, that material HAS to go somewhere. I firmly believe that in this case, boolit deformation was taking place.

So what I believe is that "Sizing For Best Accuracy" depends mainly upon what the dimensions are in your firearm. If the throat and boreform are grossly mismatched (for cast lead) then possibly you're not going to enjoy much more than average accuracy regardless of sizing for either groove OR throat.

In my 1898 Krag Infantry I size to the throat, and it takes a .310" bullet. The barrel is .301"x.3083"/.3085". At .310" it will 'just' fit. Seems to do better that way. In a 1903A1, I have no choice really. It's right on at .300"x.308" and anything over .309" will scab up in the throat.

I'm not saying that sizing to the groove+ is bad and will render bad results, because if the case and bullet are aligned, (and especially engraved) then chances are that the bullet will enter the leade straight, regardless how fat the throat. It really boils down to throat sizing merely enhances the prospect of alignment and guidence. That is, if the throat is correctly dimensioned.

What we're not talking about is caseneck concentricity, brass thickness (bending like a banana under pressure), or the firing pin driving the case toward the extractor.

.........Buckshot

Sky, you will get fins on the base no matter what you shoot. If you fit to the bore and the rifling is deep, the metal still has to flow past the base. I fit my revolver bullets as close to the throat as I can. The problem with too oversize a bullet is that the grease grooves are mangled shut, perhaps trapping much needed lube. This of course also increases the size of the fins. I shoot BPCR also and everyone talks of pristine bases being neccessary. Pushing bore size bullets through the bore shows very pronounced finning. Using an LDPE wad under the bullet helps reduce this finning and a gas checked bullet really helps. But you have to accept it as a fact of life unless you want to shoot a smoothbore.

After four decades of shooting cast bullet, I have about come to the conclusion that there is no formula that will work each and every time. There are so many variables (bore diameter, groove diamter, throat diamters, number and style of rifling, twist, bullet shape, alloy etc. etc.)that formulas just don't seem to work. Sort of like trying to nail jello to a tree. Here are some of my presumptions that I follow.

1. The less bullet sizing the better, but .003 is the limit.
2. Bullets should be the same size or .001 under throat size.
3. Bullets should be groove or over, never less than groove diameter.
4. Bore riding part of bullets should be spot on or no more than .001 larger than bore.

Very seldom do all of these things come together, but I try. My .30 cal starting sizing diamter is .310. I then adjust from that point, but most often .310 gives good results. With the Krag, which have large bores, I start with .311. Most modern commercial 30 cal rifles will choke on .311 bullets.