Thanks for the information. I was concerned about just such a build up in the chamber neck area. I'll start leaving it in place. That GP-11 ammo shoots well in my rifle.
I want to handload for it, but have found boxer primed 7.5X55 cases a bit hard to actually buy. The places that do have them seem to always be "sold out, waiting on another shipment" when I call. I did buy one box of the Hornady soft point loads but haven't shot any of them yet. They are a bit pricy for plinking.