Bigdog, the reason for the accuracy issues is reciprocating mass.
The piston guns have a chunk of steel attached to the barrel that in theory will change barrel harmonics, move around under recoil, etc.
Also, most guns made with piston systems are more "service" oriented and the ones that I have seen, a majority have chrome lined bores. Usually a chrome lined bore, while better for durability is not so good for fine accuracy.
Whatever the case, in my experience, the best of the DI guns shoot better than the best of the piston guns. The best of the piston guns are good, don't get me wrong, and my SCAR is more accurate than my FAL or some of the AR's (cheaper ones and the frankengun...) I guess my HK is a little better than most in that its about a 1.5" rifle at 100 with the few loads it likes.