It's a micro-groove.
I happen to like MC barrels, and you can actually shoot cast lead in them with or without gas checked bullets. Just use very hard lead in the 18 BHn range or better if you don't run a gas check.
The downside to MC seems to be when people shoot paper patch or cast boolits but don't make the lead hard enough. Then they will say that MC won't run properly but it's just like shooting cast in glock OEM barrels. If you cast 'em right, you can run 'em without a problem.