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After purchasing slide fire stock I figured I would need lots of ammo to accommodate my new AR's rate of fire. I found some federal green tip 62gr at a good bulk price. I'm now told that I should have a 1in 12? (I think that was the rate) and that my Olimpic arms PRC 1 carbine probably has a 1in 9 and is suited for the lighter 55gr bullet. Also I looked on my barrel and found no markings indicating rate of twist. Please enlighten me. The only mark on barrel is 5.56.
The longer the bullet the tighter the twist required. If your twist is too slow the bullet will tumble. The early 55 grain bullet used a 12 inch twist. The later 62 grain bullets need a tighter 9 inch twist and if you use tracer you need a 7 inch twist because the bullet's longer than ball. You can find your twist by using a rod with a brush and inserting from the rear with the rod out the muzzle. Mark the rod at the top and pull it until the mark has rotated once. Measure from the mark to the muzzle and you have your twist. I suspect it's a 12 inch twist if it's not marked.
Your AR most likely has a 1-9 or faster twist rate and will stabilize the 62gr green tips just fine. The 1-9 twist will reliably stabilize bullets up to 70 gr, boattail or flat base. Anything heavier generally requires a faster twist of 1-8 or 1-7. About the only AR's that have a 1-12 twist are the early M16's that are still around or the modern long barreled variants used for varmint hunting and were designed to shoot bullets of 55 gr or lighter.
Thanks to both for the outstanding info. Apparently I had it backwards or was told it backwards. I did just get off the phone with Olympic Arms and was told though I will confirm with above test that I should have the 1 in 9 twist which I now understand is what I need for the 62 gr ammo I purchased.
A funny thing I also learned.... my Oly Arms PCR 01 stands for Politically Correct Rifle.
That and the T-Shirt that says ATF ( alcohol, tobacco and Firearms) "Who's bringing the chips?" leads me to believe that gun owners are funny as well as responsible law abiding citizens.
In conclusive as I just tried and the .22 brush is old and worn and started to twist but never made it around after 16 inch pull. I've been using a bore snake so will have to buy a new brush if you think that may be voiding the test result.
The April 2012 issue of Shooting Times has an excellent article by Dick Metcalf on Twist rates and accuracy. Anyone with an interest should check it out.
You need a good brush so it bites well. Tighten the rod and work carefully. You'll only have to do it once.Quote:
Originally Posted by DaveN
You will be fine shooting 62gr ammo out of a 1/12 twist barrel. It is not optimised for that bullet weight but there is really no need to change the barrel if you are just blasting off rounds with a slide fire spray and pray stock. I have never seen a 1/9 barrel that is ummarked. The 1/9 barrels came out as the bullet weight moved up to 62gr. The current military 1/7 barrels are optimised for even heavier longer rounds used by the military (72gr, 75gr) The 1/9 barrel seems to be the middle of the road if you want to shoot ammo of various weights. The 1/7 barrels are all the rage because that is what Uncle Sam uses in the current M4s. I have both the 1/12 Colt 20" barrel and a Rock River 1/19 16" barrel. Most of the ammo I shoot is either cheapest 55gr or surplus 62gr green tip. I don't really see much of a difference unless I really try for utmost accuracy at long range from the bench. I think the accuracy/stability issue will be more of a quality of ammo manufacture than barrel twist rate in your case.
I see now from your reply you have an OLY barrel.
I tried 62 gr bullets out of the early 1/12 barrels in the 80s. The bullets went full broadside keyhole at 25 yds. I can't imagine what accuracy you get...