Hard Cast Bullets In A Polygonal Barrel

[willy3]

I have a friend who went to Glock Armorers school. They never mentioned it to the people they were training. Must not have been an issue...

 

[AMProducts]

I must respectfully disagree with two points made here...the first being that polygonal rifling does not lend itself well to cast lead bullets, and second that flat or bevel based bullets will not deform enough to seal the bore. I would stipulate that if you're not sealing the bore, no bullet (lead or jacketed) will perform particularly well in any barrel, polygonal or otherwise.

I use bevel-base bullets exclusively and those that I've recovered show the base is flattened and fully engaged with the barrel rifling. The 20Kpsi (or CUP) you state is approximately correct for BHN 15-18 lead hardness and is far from excessive in any handgun caliber I'm aware of....45ACP being probably the most limiting that I reload for.

I think you read into that paragraph a bit more than I intended. Lead bullets will strip if you push them too fast no matter what kind of barrel you have, the problem is, the characteristic curve where this happens happens at lower pressure than it would in a convention cut or button rifled barrel. This is why cast bullets can't be used in rifles over about 2000FPS, the forces just become excessive.

The opposite is also true, harder bullets (jacketed, solid copper etc) require significantly higher forces to perform well because of the harder materials.

The trick to loading a quality round of ammunition is keeping the bullet in the pressure curve that it likes for that particular firearm, too hot or too cold ain't right, no matter what the other circumstances.

Further, .45ACP is much more forgiving, as the larger diameter gives more surface area for the rifling to bite into, as the diameter drops, the surface area drops geometrically, thus a 9mm is more likely to strip than is a .45. I know, I had this rather frustrating problem with a batch of plated 9mm in some of my first lots of 9mm ammo... the rounds simply would not shoot accurately, I had tested them for pressure and velocity, but when I went to shoot them they were keyholing all over the target. Talk about frustrating! The problem ended up being a fast powder, that would cause stripping during the initial burn, even though it didn't result in much of a leading issue, the bullets were just jumping the rifling and going wherever physics took them after that.

Also, you are quite correct, the Lyman Cast Bullet Handbook is a great resource for anyone thinking about getting into casting, I bought a copy some years ago, it's now badly dog-eared and in danger of falling apart and full of notes. It is a fantastic book when it comes to issues of making bullets.