Roger. i've just done the math. You have barrel walls, assuming the bore is in the centre of the barrel, and your rifling is .012" deep, of .23175. I don't have enough experience with percussion drums to make a comment of it, but I will anyway. If your threads are National Fine (3/8 x 24 or 32)and good, and the drum is supported well by the lock plate, the scenario should be ok. With 24 TPI you will have a minimum of 5.56 threads engaging the barrel steel. With 32 TPI that increases to 7.4 minimum threads in the barrel.