Leading at the crown?

[44Blam]

I've got a GP-100 in 10 MM and am pushing a 187 grain plain base boolit about 1250 fps. It is PC'd and sized 2 thousandths over grove size. There is no leading in the barrel, just little deposits in the crown of the barrel. It's easy to remove, I just scrub it off with a brass brush. But what causes that?

[tomme boy]

You gun powder is one cause.

[Harter66]

Are you sure it's not antimony wash ? Otherwise following the thumbs it's lube failure .

[Iron Whittler]

Would need to know powder, powder wt., gas check or plain base, man. of pc powder(hf, smokes, etc.) lead hardness. A lot of variables.

[RU shooter]

Crown area may just have some minor machine marks that's catching some bullet residue ? If it's not effecting accuracy carry on as normal

[sutherpride59]

Crown area may just have some minor machine marks that's catching some bullet residue ? If it's not effecting accuracy carry on as normal
I’m not sure I completely agree with that, if it starts to build up over time it MUST affect accuracy. The crown is what evenly distributes the expanding gases to the outside world and any lack of uniformity creates a distortion at the base of the bullet at the moment it leaves the barrel. If you could post pictures we might have a better idea.

[ReloaderEd]

Measure the inside cone diameter with a good micrometer and finished bullet size or simply insert a cast bullet into the cone. that will tell you if you need to reduce the cast bullet size. I have one GP100 in .357 Magnum, it does the same thing with 38 special bullets but no problem with .357 Magnum rounds.