Drums can be tricky, but I would use a 5/16 by 24 thread drum. The best way to install them is to get the blank drum and use the tool TOW sells to drill the nipple hole, other wise be very careful about not overtorquing to align to the hammer and messing up the threads. It is better to have one a slightly loose and then solder it in place, ideally with a silver solder, but I have used 50/50. Remember to measure the walls and file off enough so that it does not stick through and interfere with cleaning. Use the top and bottom of the holes to gurantee the most thread contact. A patent breech would be more reassuring.
DP