CUP and PSI
There seems to be a bit of confusion on CUP and psi around here. Psi is a more modern measure of pressure. CUP and LUP were the standard for many years, and you will find references to it in handloading books for some time to come. CUP and LUP are copper and lead crusher pressures respectively and are actually the same measure of pressure, they are interchangeable. Psi and CUP/LUP are not interchangeable, nor is there a conversion factor between the two. CUP is a caliber specific measurement. The same CUP in one caliber is not the same as the same CUP in another caliber. 52,000 CUP in a 6mm Rem is not the same as 52,000 CUP in a 223. The psi difference between the two is 10,000psi. I know this makes no sense so just check out the attached table. It came from a Hercules handloading pamphlet, it?s not BS.
You can?t really say how much pressure an action can take, you need to calculate bolt thrust. The amount of reward thrust placed on the locking system is what determines what an action can take. When fired, a case pushes backward in response to pressure like a hydraulic piston. The total amount of reward thrust is determined by pressure and area for the cartridge base. Would you rechamber a crappy old 1 MK3 Enfield and rechamber it to 45-70, 45 ACP, 7.63x39 or even .223 Rem? Probably not, but lets do some quick math. To calculate bolt thrust you need to know the max cartridge pressure in PSI not CUP/LUP, and the cartridge case base diameter, which is the diameter of the case above the rim or extractor groove, not the head or rim diameter. You can find both in a good loading manual such as Speer or on line at Ammo Guide. Using the case base diameter, find the area of the case in cross section, which is what you see if you stare down the neck of a fired case. The formula is (radius) squared * 3.14 or you can use (diameter) squared *.785, either will get you the roughly the same answer. Once you get the area multiply it by the max pressure in psi not CUP. This is the max amount of rearward thrust placed on the locking system in pounds. Here are the case dia and max pressures of the cartridges I mentioned above.
Cartridge Case base dia Max pressure Rearward thrust lbs.
303 British .455 49,000 7963
45-70 .470 28,000 4855
9mm para .391 35,000* 4200
45 ACP .476 22,000* 3913
223 Rem .375 55,000 6071
7.62x39 .438 45,000* 6777
* pulled these off the web, couldn?t find max in psi in my books, only CUP.
Take a good look at the cross section area and max pressure columns, there is a relationship between them and bolt thrust. If the pressure and area are higher then the total thrust is higher, reduce either and the thrust is lower. The 223 has the highest pressure but not the highest thrust. The 45-70 has the highest area but not the highest thrust. Note that the 45ACP and 45-70, have similar head diameters and pressures, thus they produce similar (sort of) rearward thrust. Also note that the 7.62x39 actually produces a higher thrust than the 223. Of interest to those doing pistol caliber conversions is that the 9mm actually puts more stress on the locking system than the 45ACP, but in a blow back system that?s a whole nother can of worms. By the way, As far as I know the 1 MK3 Enfield has been reliably chambered in all of the above cartridges except 223(case head is too small for the bolt face).
I also worked out the bolt thrust on the following cartriges for reference.
Cartrige Case base dia Max press psi Bolt thrust in lbs
7.62x39 .438 45,000* 6777
8mm Mauser .470 35,000*Saami 6069
30-06 .470 60,000 10404
308 .470 60,000 10404
If you think I?m full of it (I would be doubting this too) go over to the Roderus Custom forums and look in one of the 50BMG construction threads (don?t remember which one) and check out how to do the numbers yourself. I don?t want to come across in the wrong way, I?m not trying to piss on anybody?s parade, I?m just trying to share what I have learned from others, so that we can be safe, I like my face the way it is. The point I am trying to make is that max psi in a cartridge does not determine if it is safe to shoot in a specific action, bolt thrust does, and that psi and CUP are not the same thing. If you still believe that pressure is the sole determining factor of what and action can take, consider this. The max pressure of a 22lr is 24,000psi while a 12ga is only 11,500psi; a 12ga action is a hell of a lot stronger than most 22lrs.