+1 for 18" SPRs. The Mark 12 Mod 1 (and Mod 0) are built to be effective out to 800 meters with 77 gr. SMK bullets. (MK 262 ammo type.) You can get this stuff from
Black Hills Ammo.
I have an A2, 20" and a Mark 12 Mod 1 and while it IS true that the A2 is more accurate at shorter ranges (100 to 200 yards, anyway) due to its National Match Bushmaster
heavy barrel, which is a FINE pipe, the Mark 12 is very nearly as accurate at shorter ranges but comes into its own after 300 yards. For these longer shots, it is unquestionably the better rifle, at least with the heavier bullet.
Worst case scenario, the Mark 12 still beats 1 MOA, easily. The A2 NM barrel can hit 1/4 MOA groups reliably but only at shorter range. It opens up disproportionately beyond
300 yards as the lighter bullet slows down a lot and gets moved around by breezes pretty easily. Or at least, that's what I'm seeing for myself. YMMV.
I'd definitely put in a vote for an 18" SPR. It'll do everything the A2 will, and stay effective at longer ranges, too.
CJ