Yes, the powder will burn. The bullet you speak of isn't that hefty. The barrel steel isn't the issue, but the nature of you breech plug and ignition system is.
.50 caliber, 800 grain patched bullet over 180 gr Swiss 1-1/2 Fg works.
.56 cal, 1300 gr patched bullet over 200 gr Swiss 1-1/2 Fg works too.
Wouldn't suggest such loads for conventional BP rifles but underhammer or sidehammer guns with platinum lined nipples or sealed ignition handle it just fine. It helps with recoil if they weigh 25-40#, give or take.
