The two best things I've done for my AR's was one piece gas rings (Three turns) and a Dewey lug cleaning tool with Nylon brushes.
The one piece gas ring doesn't carve up the bolt carrier as the stock rings expand and the tails strighten out,
And once the one piece gas ring seats, it's a VERY good seal, no more failuers due to gas rings lining up and letting too much gas leak out of the piston area.
VERY good for keeping sand, carbon, ect. from collecting in the bolt and scratching up the rings or bolt carrier.
The Dewey lug cleaner is nothing special with the exception it's long enough for most pistols also,
And it comes with bushing for the upper receiver....
The nylon brushes don't mat down, the ends don't dull and refuse to cut like the metal brushes do,
And they don't scratch up my lugs like metal brushes do.
Just one man's opinion, but they have cut my failures WAY down,
And they have reduced my cleaning time by about half!
For polishing the bolt tail, I don't care to under size my bolt that much, and a piece of 'Scotch-Brite' will do what they are showing in no time just by turning between your fingers.