Hi,
I have benn struggling with the dreaded hard carbon ring that forms at the lead right in front of the case neck mouth. As it forms it causes pressure increases which alter your velocity SDs among other stuff.
I have found 2 ways to minimize it.
First, trim brass to .001" under max SAMI length spec. Short brass trimmed to minimum length is the biggest contributor.
The way I get rid of it is twofold.
I start by doing this EVERYTIME I shoot the rifle so it never has a chance to build up layers . When I get home from a range session I take a bore mop and dip it into my gallon can of Berrymans Chemdip carburetor/parts cleaner, and insert it in the chamber so it saturates the area the ring forms. I then walk away and leave overnight. Next morning before I leave for work I take a bore brush and insert it in place of the mopand by hand, twist it about 20 turns. I then repeat the thing with the bore mop. When I get home from work I remove the mop, repeat the brushing and clean the rest of the barrel. The Chemdip really softens the carbon and lets a bore brush clean it out, but is harmless to steel. You can use abrasives like Iosso,but I like letting the chemicals do it. This is the only way after 40 yrs of experimenting with other methods I've found to let me get the throat back to squeaky clean with no hard carbon. Slip2000 carbon remover also works with the bore mop, but it takes longer.