Why would you shoot Black powder in a gun recently proofed for a nitro? That makes no sense at all. The Birmingham Proof house told me a gun I had which was reproofed for Black powder should be fine for 2 1/2” nitro loads. Sane loads would be fine. It seems there was not significantly more pressure in nitro proof in the 1930. Then gun is still well in proof. Guns can’t tell what powder you use only how high the pressure is. So keep the pressures down to 7,000psi, or if really conservative 5,500 psi and shoot the darn gun. There are a lot of sub 7,000 psi loads to pick from so why bother with Black?