where can I find out more if an enfield action will handle the .375 win, I've been looking at pressure ratings and Im starting to think either I need to get an ishapore action or go to an alternate action
.303 brit 45,000 CUP, 52,900 CIP
.375 win 52,000 CUP, 64,000 CIP
7.62x51 NATO 52,000 CUP,
.308 win 64,000 CUP
Any links to what the no4 action will handle or are they just stamped as per the 303 brit?
Another consideration is the amount of backthrust.
Load on the chamber walls is a hoop stress, and is not related to the action. It is the barrel which contains it.
Backthrust, on the other hand, is very relevant to the action.
You can calculate the amount of backthrust by considering the area of the inside of base of the cartridge case, and the peak pressure applied to it. A smaller cartridge case at higher pressure may generate less backthrust than a larger case at lower pressure.
The older Thompson Center single shots were made in high pressure small cartridge chamberings, but only in lower pressure larger cartridges. The design could not handle larger diameter cases at higher pressures.
Get out your calculator, and compare the backthrust generated by .303B vs .375W.
As far as the Ishapore 7.62m action is concerned, there are reports that a better grade of steel was used, along with a modified proof testing regimen.
It has also been reported that no special steel was used, but only proof testing was altered, to eliminate bent receivers.
It was a proof load with an oiled case that would bend SMLE receivers - because of the increased backthrust. Eliminating the oiled proof load solved the problem.
In the days when 7.62mm No. 4 rifles were used for target competition, shooters learned to keep their ammunition dry when shooting in the rain. The change in backthrust resulting from firing a wet round affected the vibration pattern of the action and barrel, resulting in a flyer from the group.
Ongoing supply of factory .375W ammunition could be problematic. As you have noted, distribution is spotty - that isn't going to improve. Perhaps handloading could be a significant factor in the future of the .375W.