This site was pretty good (and hoping reliable) for pressure specs.
http://kwk.us/pressures.html

I ended up using the rim diameters and case diameters as I didn't have the internal measurements but this will hopefully be apples and apples with F=pxA

Here are the back thrust numbers for a number of cartridges found in the Valmet with rim diameters.

9.3x74R 10290 for 49k

30-06 10620 for 59k

12ga 6000 for 12k

450-400 12000 for 40k

If we use case diameters instead:

9.3x74R 8497

30-06 10620

12ga 9540 (looked at 19k proof)

450-400 9292

I don't think that I messed up any of the math and the case diameter numbers seem more reliable (and in line) with cartridges chambered in doubles.

Good luck.

Tom

Tom