I see your points & reasononing, Dugaboy. As to the normal extractors purchase on a large rim as opposed to the small pawl of a rimless design - point well taken.

As to the straighter sided case shape causing problems at some point if a weak extraction rifle, I still see .005" to .010" taper per side as being quite enough due to the additional .001" contraction of the brass all the way around.

I've tested the extraction of a .257 IMP case in a Mauser bolt rifle that had NO extractor- back in 1978. The rounds fell from the chamber when the muzzle was raised. There had been no primary extraction. I've since (last year) checked that initial test of extractor removed with both a 9.3x62 and my .375/06IMP with identical results. In the 9.3x62 test, the case matched a normal Ackley Imp. case with .009" taper per side in my factory chambered Mauser rifle, while my .375/06IMP has even less taper, only .005" per side. In both, the rounds fell from the chamber when no primary extraction was used (no extractors). This showed me, they'd probably work well in any rifle with poor extraction.

As to pressures developed,I can only speak to velocities. The 9.3x62 runs 2,519fps with 286gr. Normas and 2,675 fps with 270 Speers. My .375 does 2,740fps with 270gr. TSX and 2,470fps with 4 different makes of 300's. I submit pressures "are up there". None of these rifles has expanded a primer pocket in firing these loads, many times each.