leave a "step" where the end plug meets the tube. I have another R10 receiver from a R10 parts gun I bought a while back and the OD of my existing R10 end plug doesn't match up very well with that tube. It seems that the R1 & R10 style receivers are machined as a "matched set" so I prefer to maintain the geometry.
Actually, I believe that my often "screwing and unscrewing" of the cap under spring pressure literally "lapped" the threads till the fit became sloppy.
I noticed that the worn end cap of my HW77 also had a "not very noticeable but could be felt" misalignment between the OD of the end cap and the receiver and a single coat of the baked on finish was enough to both snug up the fit and align the end cap with the receiver OD. The issue with the HW77 receiver is that the milled scope rail goes from the end plug into the receiver tube so the "felt misalignment" affected the alignment of the scope mount. Since the R10 has an applied scope rail this isn't an issue, and I guess that the minor misalignment of the front and rear of the HW77 milled grooves wasn't MUCH of an issue since I used a one piece mount, however I preferred a snugger fitting end cap that FEELS properly aligned.