iloco: If the drum was installed entirely forward of the breech plug threads, in other words, does not interfere with the removal of the breech plug, you are in luck. Remove the breech plug, and with a ball stone on a Dremel rotary tool, grind off the offending protrusion down to the barrel. This part of the drum is not making the joint between the drum and the barrel any stronger, and is a nuisance. In an ideal situation, the builder would have used a barrel with sufficient wall thickness to completely house the full length of the drum's threaded journal. Original rifles traditionally have thick walls at the breech, but modern barrels, in order to make them lighter for today's inferior man, are often much thinner. I site an example that I have experience with in the first years of my 'career'.
I was handed a 13/16" x .45 calibre barrel and a 5/16" x 24 threaded drum, and asked to make a rifle around it. Long story short, the drum and the barrel parted company not long after the client took delivery...my fault entirely, of course. That was the last time I installed a drum in any barrel, and the last time I worked on a rifle with a 13/16" barrel.