When I install a drum, I want the threads to be as close to the breech plug face as possible without interfering. The actual drum threads won't interfere as you will trim off the drum inside the barrel and finish mirror smooth or else you will get gunk build up and constantly tear your cleaning patches. You don't want the drum threads to interfere with the breech plug threads either. With many drum installations, you are somewhat constrained because the drum location is actually determined by either the drum cutout on the lock or the hammer geometry. Hope this helps.
