Greydog... Equal pressure & valid comparisons? SAAMI for 30-06 & 280 is 60K. It's 65K for the 280 Ackley & the 270. The sharper shoulder in itself does not make brass withstand higher pressures. Brass is brass & is still the weak link in the chain. If you can find a copy of Ackley's book you can read his test results for yourself, but you might read something you dont want to hear. Paraphrasing from memory he says that the improved case, upon firing, grips the chamber walls better than a tapered case, thus reducing the bolt thrust. If I remember right there are a few tests to destruction of various actions in both standard & improved cartridges. I think there was a lever action chambered to 30-30 Improved tested with no locking mechanism, but am too lazy too dig out the books this morning. The day's a wastin' & the grass needs cuttin'.
MedRiver, I think you're short changing yourself with a 22" barrel, but your results will be interesting.
Enjoy your Ackley Bob. Dont be afraid to push the envelope a bit with care & within reason. Hoping you got a high # 03. IMR 4350 was the powder when I did most of the development. The 4831s filled the case well before before pressuring out. RL 19 & 22 were showing promise with 150s & 180s respectively. The new (plastic) jug of IMR4350 I got doesn't develop anywhere near the same speeds as the old (metal) can. Very disappointing. With the newer powders, RL17 has given good results with 150s. The biggest gains were realized with 150s & 165s so far, but with the plethora of newer slow burners, who knows? I will be trying some RL23 soon with 150s & 165s & may try RL26 with 180s.
I have a copy of Ackley's book which I got as a Christmas gift 56 years ago, when I was fifteen. I am very familiar with his theory on reduced bolt thrust and believed it until I was 16. It was then that I separated a case with a hot load in my Lee Enfield. At this point my thought process was like this; if the case separated, the forward portion of the case must have been gripping the chamber wall sufficiently to remain in place while the base moved back. If this was the case, why would gripping the chamber wall even more tightly reduce bolt thrust? If the 303 british case (plenty-o-taper) could hold on tightly enough to not move back, what difference would the improved case make? At the time, I decided the difference might have been in Ackleys use of the 30/30 in his tests. Maybe 30/30 brass behaved differently.
It was a few years later that I started gunsmithing myself and was able to try testing some of these things on my own. I am convinced, practically speaking, case taper makes no difference in bolt thrust though it may have some effect within a very narrow range of pressure; with this depending on brass thickness and hardness. At high pressures, ALL cartridges grip the camber walls sufficiently well to allow the head of the case to blow off if not adequately supported. In addition, the amount of deflection of the bolt at a given pressure most certainly comes into play. A strong, front-locking, bolt action will deflect very little at 70,000 PSI. So little that brass stretch is largely meaningless. If the bolt face deflects by .001", the amount of pressure required to stretch the brass that much is insignificant.
If unsupported, brass can only do so much. Primer pockets expand at the same pressures regardless of shoulder angle or case taper. Rimmed cases which are fully enclosed in the chamber, as in most single shot designs, may be able to withstand higher pressures than rimless designs but I'm not convinced enough to say it is so. GD
