the reason i ask is because a friend has one (i do also, same kind, no probs here) on a 300 win mag. it started to shoot funny. upon inspection of the break we found what appeared to be fire erosion on the inside part of the muzzle end along with a lip of what appeared to be once molten metal. the erosion seemed to be in the same pattern as the riflings. he took a dremal and carefully polished it out. they were deep that enouph i thought it need a machinest. shoots great now. the barrel only has about 250 rounds threw it. my friend has a bore scope and the barrel looks fine.
the muzzle break and barrel are not a cheap nock-offs and were professionally installed
What you seen was in fact erosion and displacement of metal due to the blast/erosion and pressure that the baffle sees.
The very first brakes I made of 416SS about 21 RC showed this on a customers gun in 300 win mag burning H1000 happened after 300 rounds. The week before that a new customer brought in a SP with another smiths brake that showed the same erosion.
That necesitated heat treatment of that batch of brakes to toughen them up. Unfortunatly about a half dozen made it out of the shop in that time. Since then I use a hardened 416 stainless That is rockwell 28-30, Has not been an issue with this material.
As the first, and often all baffles in the softer brakes gets beaten up by the blast furnace of gas, it actually can turn in the leading edge of the clearance hole, constricting that hole and when bad enough cause the bullet to drag on a portion of the bur, ruining accuracy.
Also even on the hard brakes, if the tru hole was drilled instead of bored by the smith, he could inadvertently created a raised bur on the baffles leading edge that in turn could either fracture off or roll in.
A good way to check is shine a light in the clearance hole looking for the bur or fibers of cleaning patch that caught the bur.
To remedy the problem put a couple tounge depressors in the first baffle to protect the crown. use a brass rod in a drill with a split in the end to hold a stip of 320 emery cloth. Spin the dowel in the clearance holes (entering oposit the bullets exit)and the paper will work off the bur, as these burs tend to be brittle and fractue off relativly easy.
This occurs more rapidly in SP due to the shorter Barrels