I've used Gamebore Pure Gold (paper) 2 1/2" 12's on pheasants with good results. 1 1/16 oz Brit 6's (which are about 6 2/3 American size).

Re pressure, if they're 2 1/2", then they'd have to fall within CIP specs for standard proof guns. Maximum service pressure of 10,730 psi as we measure it on this side of the pond. But likely a good bit lower than that.