I am planning on buy/building an AR15 to shoot in DCM matches. What is the best rate of twist for this? I have never shot a DCM match but qualified several times on the Marine Corps known distance courses, shooting out to 500 yards. I do not plan on shooting any distances exceeding 600 yards. The RRA upper I am planning to buy has a 1:8 twist, but according to the ammo-oracle this is to stabilize heavier/longer rounds. I do not currently reload and will probably be firing factory ammo up to 62 grains or military surplus. Will the 1:9 twist be better for me. Any ideas? Thanks.

If you are gonna be shooting long range 500yd you might want at least 62+ grain ammo even try 75 grain if you can get it, a 1:8 twist will stabilize that round. GI rifles are designed to shoot 55 grain ammo so they come in 1:9 or 1:10, Please correct me if im wrong. As for military 'Surplus' I assume these are 55 grainers they are cheap to shoot, but for matches use a good Match grade ammo like Black Hills brand 62 grain or hand load 75 gr. 55 to 61 grain use 1:9, 62 and up use 1:8, Im on a Navy CMP team so I just gave you my secrets, cya at the match.

Gaspain

The RRA DCM/CMP upper is a fine choice IMHO, I own one and this upper will shoot anything from 55 grn. mil-spec M193 up to factory (Black Hills) 77grn. match and even the heavier JLK 90s.

Believe you'll be making a better choice sticking to the faster twist for XC type matches.

Mike  

Thank you gentlemen.  Your expertise is much appreciated and I plan on going with the RRA upper.  

1x14 twist was the original twist for the M16.  It will just barely stabilize 55 gr, but no heavier.

The M16A1 was modified to have a 1x12 twist, which will stabilize 55 gr, but not 62 gr.  Of course, 1x12 will also stabilize 45 gr bullets, too.

Aside-
During Desert Shield, it was found that many National Guard Units were still equipped with hand-me-down M16A1's, with 1x12 twist.  Since the ammo in the supply chain was M855 (62 gr) there was a big panic, as the A1's had no accuracy with the M855 ammo.  The unstable bullets could be seen to keyhole in the targets.  Fortunately, many small AR15 manufacturers, such as Quality Parts (Bushmaster) came to the rescue and helped supply M16A2's.

1x10 will barely stabilize 69 gr bullets.

1x9 twist will stabilize 45 gr, 50 gr, 52 gr, 55 gr, 62 gr, and 69 gr  bullets.

1x8 twist is intended for 69 gr match and heavier bullets, but will certainly stabilize lighter 45 gr to 64 gr bullets.

1x7 twist is the standard military twist for the M16A2, and is unnecessarily fast, designed to stabilize tracers in arctic conditions.  Well, next time I go to the north pole, and want to shoot tracers, I will make sure I have a 1x7 twist rifle.

1x9 twist will satisfy all but bench rest shooters who use the very heavy 77 and 80 gr bullets.

Over stabilizing a bullet will not make it more accurate.

1 in 7....good one!
Good info from A_FREE_MAN