I have a relatively new Armalite DCM (about 1000 rds).  It was broken in with 3 10-shot strings of the finest-grit Tubb final finish bullets and is cleaned about every 40 shots, and treated with Parsec 0.3 micron moly.  I cannot make it group better than about 2 MOA.  I am shooting

-- military brass sorted by year
-- WLR primers
-- 24.5gr Varget
-- Sierra 69 moly and nude; OAL 2.26"
-- Sierra 80 nude; OAL 2.45"

About 1 primer in 50 gets perforated, and there seems no relation between whether the bullet is molyed or copper when the primer malfunction occurs.

I admit to shortcomings as a benchrest shooter, but can generally get 1 MOA from my friends' custom and bushmaster DCM rifles.  

Any suggestions as to technique or, particularly, as to loads?

I use Hornady 175gr moly-coated factory ammo.  With that, I can put 5 rounds in 1 inch at 100yds.  With a scope.  The 155gr Hornady's also work well, which suprised me.  I can't get close to that accuracy with any other brand.

Post this question in the General Forum under "reloading or "accurate AR" specifically.  The members over there will steer you in the right direction.