I shoot a couple of ARs quite a bit for practical rifle and 3 gun competition...at least two matches a month, and previously preparation for all of the Ft Benning matches, and prior to 9/11, 5 of the North Carolina Tactical matches
I have a couple of friends in the industry, and have one of their reliability kits in my bolt ( extractor "O" ring, and a high temp ejector spring)...they advised that if you do not see any cracking of the bolt locking lugs within the first 500 rounds, you should be good to go... I shoot Russian ammo, and had to replace the continuous gas ring with a traditional 3 ring setup to make it through a match
I got a bad lot of Win white box that was excessively hot and flame cut around the firing pin hole...I replaced that bolt with a chromed one
the only problems I've encountered were with a Colt non shrouded (firing pin) carrier that caused the hammer to peen the shoulder of the firing pin, and bolt cam wear... I replaced the carrier with a shrouded M16 carrier from BCM and a hardened chromed cam pin from Smiths
I clean the chamber with a "T" handled brush, and clean the carbon from the bolt each match... I use Mercury Outboard Quicksilver racing oil and run the bolt and carrier wet...will probably switch to the Slip EWL when I can get some...I degrease the chamber area with brake cleaner
the lowers are pretty much identical.. Rock River NM triggers, ambi safety, VietNam era A1 stocks... one is intermediate gas system...both have full float tubes with just bottom rail as I use a Knights forward grip for a monopod....1.5x16 and 4x32 ACOGS...
enjoy your AR and check out any practical rifle or 3 gun matches in your area