If you alter a chamber length it will be out of proof ,if a gun has multiple barrels and you lengthin the chambers it will be out of proof .buy a modern gun with modern steel proofed for modern ammo. a European 2.5 gun proofed for 1 1/8 oz cip loads is not proofed for 2 3/4 sammi ammo .no matter how much you beat the dead horse it's still dead