How to clean heavy carbon buildup? Chisel or grinder?

I use a paper towel, and just wipe it off.

Here's the deal..... get on the Brownells website, and order some SLIP2000 'carbon killer'. It's a green liquid, comes in a plastic quart sized container. Kind of smells like oranges.

Take the parts you want to 'carbon kill' and drop them in the goop for about 5 minutes. Pull 'em out and wipe the carbon off with a paper towel. BE WARNED HOWEVER: carbon killer takes off ALL the oil - you gotta oil it afterwords or the parts will rust. This works tremendously with things like AR15 bolt carriers and bolts - for oil I hose 'em down with Birchwood Casey Sheath (the aerosol gets into all the little cracks and such) then wipe off the excess with a towel.

For parts that are too big to stick in the carbon killer (like the gas piston) I've got a two-step process. First, get the majority out with a drill bit like I do with my M14 gas pistons. Then Hoppes Elite gun cleaner (the squirt-pump) will work the rest out FAST. Not as fast as the carbon killer, but quick.

For a lubricant, get a quart of Mobil 1 synthetic oil. The whole purpose of that oil is to keep carbon in solution so it can be filtered out (reduces carbon deposits in engines). It smokes a bit at first, but on things like AR15 bolts, unless the oil dries out - more like migrates away - the carbon can be wiped off.

I got 'clued in' on the Carbon Killer by the armorer at Gunsite, the Mobil 1 from the Navy's National Match armorer, the Hoppes Elite from a barrel maker. They all work.

Hope that helps!