Max blast chamber pressure calculated using numbers below is about 3500 psi. Does it look right? (vessel is not sealed, so pressure should be lees then calculated)
.308 18" barrel volume 1.27
1.24x3" blast chamber volume 3.62
2.85 times
10.000 uncorking pressure

It looks like 10,000 to 11,000psi. I have always heard it was 3,000 to 5,000.
11,000psi is pretty intense.

Your numbers are a bit off, but not terribly.

Barrel length needs to have the "chamber" subtracted. Use cartridge overall length, minus bullet length, and subtract this from barrel length. Use that as bore volume. Use QL's "Useable Case Capacity" as your constant for pMax (62kpsi) volume.

Also, your blast volume needs to have volume of mount/brake/etc subtracted as well as blast baffle's proximal geometry.

All of this aside, I still get ~10kpsi uncork and ~2.7kpsi for internal blast chamber.