Have you shot any jacketed ammo through the barrel? I wonder if the gas port still has a little burr on it or is still sharp and is shaving off some bullet material. On a 223 any burr doesn't last past the first few shots, but that's not the case with slower bullets. For fun a few years back I made a few 9x19 DI uppers. The first one put lots of lead in the gas system. I tracked it down to a burr on the gas port. Tried shooting some jacketed ammo through it which helped but did not entirely remove it. Ended up using a diamond ball dremel bit by hand to get the last bit of it. I'm shooting Lee 124gr RNTL bullets which leave tons of soft fouling in the carrier, but now there is no lead. The most I've ever shot at one time is between 350-500 rounds.