What beautiful and informative information you last two have given, on my cast bullet forum!!!
But I started it, intentionally, because it's a field that hasn't been discussed much in the gun rags, to my knowledge. It is not totally irrelivant to cast, in that too much twist does make bullets strip a bit easier at higher velocities, but it has no effect on terminal performance that I have ever determined.
Now enter perhaps a more enlightening factor. Jacket bore fouling, and rough barrels. This will primarily be concerning jacketed bullets, but definitely not entirely. Most shooters, maybe all, simply watch accuracy fall apart to give an indicator of amount of bore fouling. And, shooters who have owned several guns of one cartridge chambering know that some barrels foul bad enough to lose accuracy much faster than others.
The problem isn't necessarily, perhaps entirely is a better word, that one barrel builds fouling faster than another, but that barrel diameter if truly straight end to end will turn into a reverse choke barrel with minimal fouling. One that is tapered small at the muzzle, can build up fouling till diameter close to the chamber, where fouling builds fastest, becomes smaller than the muzzle, before it's accuracy falls off. Roughness of the bore finish isn't necessarily the major factor in fouling either, but it appears that the bore must have a straight smooth surface for the bullet to slide on, which if it contains visible shallow pockets, roughness, or whatever one wants to call it, these relieve bore friction enough that less fouling will be created than in some perfectly smooth bores. To define that a bit more precisely. When one has a roughly machined bore and laps it to a partial clean up, he knocks the high spots all to one level, but doesn't touch the low spots. At the same time, if the lapping is done with an LBT bore lap kit, the bore develops a slight taper, tight at the muzzle. Such a gun will handle lead just fine at very high velocity, and give minimal jacket fouling, in my experience, while the large at the throat area bore has more room to accumulate fouling bad enough to degrade accuracy.
Now the last curve ball regarding jacketed bullet terminal performance. When bores become extremely fouled, long beyond the point where accuracy falls off, bullet jackets can be heated to the melt point on the surface, which causes them to explode on impact with very poor penetration. Remember that when you talk to backwoodsy people who are complaining about their rifle not killing as well as it used to. I knew one hunter here who shot 14 deer from his house before running out of shells and buying a box of another brand, weight etc, one of which killed the first deer he hit. He was so dumb he wouldn't listen when I told him what his real problem was, because a new box of shells had solved the problem. ------- Another neighbor using an old Springfield 30-06 had a 150 gr bullet blow up on a deers ribs at 100 yards or so. I measured his bore and found it was fouled to a diameter of .300 just ahead of the chamber. Pressures were extreme, in that primers were near their pressure limit, and recoil was excessive!! Another local had shot 6 deer in one season without recovering one, when my wife and I visited them, with a few days of season left. I scrubbed his bore with bon ami scouring powder on a tight patch, then told him to keep a bit of 3 in 1 oil in the bore and it would kill deer just fine till he could get it properly cleaned. He shot a deer a day or two later and his bullet went clear through, killing the deer in its tracks. To my knowledge he still hasn't cleaned the bore but just keeps it oiled. (An oil film vaporizes when wedges between bullet and bore, keeping the jacket cool so they hold together on impact. It is a crutch, not a fix.) So inform others of this problem. It is far more prevalent than one would think, as I've personally known maybe 20 hunters who have had the problem..
