I wouldn't be so sure. Even with blackpowder, a 1 1/2 oz load in a 20 gauge has to have sky high pressures, I'm sure north of 10,000 psi.
It is insane that people lambasted me for wanting to shoot 1 5/8 oz and 1 3/4 oz loads in my 10 gauge, which probably only run 4,000-5,000 psi. Muzzleloaders...