Helps a lot Bramble, I was hoping you would see this and comment.

When you calculated the breech thrust/pressure on your 450#2, did you base the area on head diameter, rim diameter, or the diameter of the vessel i.e. the diameter of the inside cavity of the case at the base?

I found some factory specs of 450#2 ammo, namely 80gr cordite and a 480gr jacketed bullet. Looks like they generate anywhere from 13.0 to 13.7 ton. 13.7 ton converts to 30,688 psi. If the 30% rule is used, a "proof load" would need to generate around 40,000 psi?

I do believe this 10ga proofed at 14kpsi, should be a good candidate for a 450#2.

Regards,

Jeff