You're right there, Alan. Ellis Brown's calculations simply don't add up in his book. The method is sound, however, you just need to use the correct figures!

If you do the sums correctly, you will find that the 17,000-odd psi proof-load of a 12,000-psi 12-bore cartridge will produce a similar force on the standing breech as a .450 NE at 32,000 psi (because of the smaller head-area of the latter). Although its probably dodgy comparing the 12-bore PROOF load with the .450 Nitro WORKING load, I think that the similar force on the breech is the point that Ellis Brown is trying to make.

BTW, I bought my copy of that book when we were only getting about 50 US cents to the Aussie dollar. OUCH!!!

ps: I just edited this post as I had inadvertently used the wrong figures myself!

Should have been the 12-bore PROOF load, fixed now!