I just picked up a 20" SFAR. I noticed that there is, at least in my opinion, too little clearance between the gas block and the handguard. I like to see at least 3/32" of clearance all around the gas block, especially with thinner/lighter barrels. I just fixed the same problem with another large frame AR in 260 that was grouping poorly. I have also experienced the same with a 223 AR that had too little clearance. I suspect this is the cause of accuracy issues in these rifles. Another thing I've found with 308 ARs is that loads that work well in a 20" barrel don't shoot well in a 22" barrel and vice versa. I suspect the same is true with 16" and 20" barrels.
