Really not sure about the Bobcats, but a lot of the CVA's and the Traditions rifles had a rather strange set up. Instead of the threads on the drum just going through the barrel wall, it seems like they went completely across the bore and embedded into the far side of the barrel wall. The breachplug then was hollow and a hole was drilled into that little tube. This is why CVA/Traditions advised against removing, loosening or tightening the drum. It could get out of alignment and then where would you be? I am just not sure how well the rifle would work if you replaced the drum with one that just screwed into the barrel. By the way, the drum probably has metric threads.
John (Bigsmoke)