Siringo wrote: . . . you are most likely correct from the beginning -- your 1/16 twist is just to fast and your chamber is different (no free bore - correct). Also, since the bore is .451, does this barrel have shallow rifling similar to the 45 ACP? That would account for the stripping -- but if stripping was really the culprit, your throat area would really be leaded up. I never had any barrel leading.
Did the bullet holes look square to the target face? I can't tell from the photos.
Also, the 460 S&W seems to handle the heavy cast bullets from Cast Performance just fine at 1800 fps. However, the S&W revolver has gain twist rifling -- helps prevent the stripping.
Siringo-
The barrel was originally drilled and rifled as a 45 ACP barrel, so it has a 1-in-16" twist and I assume the standard depth of rifling (0.451) and bore diameter, although I've never slugged the bore to find precisely what the dimensions are.
Manson ground the reamer for this chamber to SAAMI specs for the 450 B. Manson has ground the reamers that Bushmaster uses to make the chambers for all their AR 450B rifles, so the chamber should be the same as most everybody's.
Bullet holes all appear square, with no oblong or oval holes as evidence of tipping and instability. This is an important point as evidence that the bullets were in fact spinning fast enough to stabilize; thanks for thinking of this fact that I neglected to mention in the report above.
Also, I can see no obvious build up of lead in the throat or in the grooves at the throat. It may well be that the bullets are not stripping. So, whatever is causing the problem at 1900+ fps does not appear be as large a factor at 1270 fps. I'll not discount any reasonable hypothesis to explain this.
--Bob
