The numbers I posted should be good for a manually operated action, I don?t know what other stress are imposed on a semi auto action. If you go over to the Roderus forums and look in the Ask a Guru section and look at the ?other 50cal designs? post (I think that?s the name) you will see a good bit of info on the stresses placed on the locks of a 50cal action. The difference is that sense these are home builds, they know the locking lug area and the material yield strength. If you can find out how strong the steel in an AK action is you could probably do the same. Actually, If you post a question in that forum you would probably get some good responses from knowledgeable people. I have never designed or built a rifle, I was just trying to explain the differences between CUP and psi. The info on the enfield I included was just an example because I have seen it chambered in those calibers.
I think one of the difficulties you would encounter are the fact that the 7.62x39 bolt and carrier are going to be a bit short for the long 8mm cartridge. They may not be long enough to hold the rounds down in the mag. That may be the reason that the 8mm bolt has longer lugs, or it could be that the longer lugs are stronger, which they would be. It would be nice if someone who has an 8mm bolt and carrier would take a pic with ? grid graph paper as a background so you could get the basic dims, and also measure from the back of the carrier to the front of the bolt, that would give you the relationship to the position as it sasses over the cartridge in the mag. The pressure I posted for the 8mm was a saami spec, It is noted in my Speer loading manual as being low so as not to cause problems with the old small ring mausers. The surplus you get from Europe is no doubt loaded to a different spec. The military stuff may be quite a bit hotter than US specs. Arsenals often load ammo for specific weapons platforms for military use, so who knows what the pressure is.
Another problem is where to put the gas block. It needs to be the right distance from the chamber in order to catch the pressure curve in the right spot, so that it has the right amount of gas to operate the system. You might want to ask the guys doing the pistol conversions with ultra short barrels there thoughts on this. The 1.5mm receiver on the Yougo is defiantly the way to go. Global had at one time mentioned using their 1.6mm receivers for ?larger cartridges?, its not on their web sight now, I guess it didn?t pan out. If someone here gets a 30-06 Siaga and would be willing to post pics and take some measurements that would help you a lot too.
The $the 64.000 question I cant answer, but I think the Yougoslavian engineers already did, they redesigned the bolt and carrier. Technicly you should use the case ID to calculate thrust, but that info is hard to find, and besides using the OD will make the thrust come out slightly higher than it really is and will give you a very slight saftey margin. And yes please check my math, I take everything I find on the net with a grain of salt. I see a lot of stuff that just doesn't smell right.
Sorry I am a bit scatterbrianed right now, but to answer your question about why two different cartriges use different amount of powder and prodece the same pressure, it depends on a lot of things. The throat length and style, bore dia, and bullet seating depth can all play a part in pressure differences. Longer throats and freebore tend to reduce pressure, and thaey are specific to the rifle not the cartrige. It depends on what the ammo manufacturers used for testing. The Speer #11 data for the 7.62x39 was taken using a mini30 which i have been told has a long throat to reduce the pressure of firing a .311 bullet through a .308 bore, yet Speer used .308 bullets in thier testing. There are alot of people shooting mil spec .311 in thier mini30s without any problems. Fire a .308 bullet in a .311 AK bore and im sure you would get a different pressure measurement. Also another reson for the powder and pressure differences in callibers is that some case shapes are more efficient at burning powder than others. If you have a load book handy check out the amount of powder required to push the same bullet to the same velocities in different cartriges. The bench rest and PPC series are very efficient users of powder, while the 30-30 and other less bottle necked designes are not. Pressure is really the result of a lot of things acting together and you have to take that into account.
Just noticed something else
"22-308, 308, 338-08, 458 win (same parrent case)"
The 458 Win Mag is not based on the 308 case, it is a belted magnum and it has a base dia of .511.