Depending on the country of origin, the original "black powder" proofs, may be plenty high enough to meet modern CIP standards, which are in the 9000-10,000 PSI range and usually explicitly marked on the box.
On the other hand, current American SAAMI standard shells run to 11,000 PSI Maximum Average, with statistical gusts to around 14,000 PSI. As John suggests, avoid any American shell that is not explicitly marked "low pressure", because if not so designated they will likely run at higher pressures, although no one can say specifically.
Rio "Top Trap" 12g 28 gram, for instance, meets CIP standard, and the published pressure is 575 kg/cm^2, or 8176 psi. I would have no concern about using such a shell in a sound black powder proved gun.
Good Luck and have fun.
Regards
GKT
