Best twist rate for AR?

[SLYoteBoy]

1:7 is what I have , all I shoot is 55gr pmc/wolf , hits what I aim at , nuff said. Im not going for quarter minute of gnats *** at 100yds.

[mrvash]

I learned alot from this thread, my AR has a WASP 1:9 twist 16" barrel and I shoot whatever I get through it (55 gr, 62 gr and 7xgr) and I'm able to hit propane tanks and 1 gallon jugs of water out to 100-400 yards easily using irons. Also, my Saiga .223 has apparently a 1:9 twist as well, and I shoot almost just as well with it.

If I had a chance to change my twist rate to whatever? Hell no, I love my 1:9 AR barrel, and I'm not just saying that cause I spent money on a 1:9 barrel, I'm just explaining my experiences and opinion. I'm sure a bad guy, zombie, paper bullseye or whatever my target may be will care what barrel twist I have! and you can bet your *** atleast more than 1 out of 10 rounds will hit it's target with my current shooting skills and do it's job to it's intended purpose with it's current scenario.

[Knife Edge]

Somebody posted a link to this website in regards to the twist rate discussion. See below for his post - I think this guy makes some great points. Read the bold section!(makes alot of sense)

from http://mscg.yuku.com

Bullets must have enough spin (RPM) when leaving the bore in order to fly true. Remember bullets begin to slow down (in RPM and velocity) from the moment they leave the bore so they must have enough spin from the start so that they remain spinning enough to fly true over the course of their trajectory otherwise if they arent spinning enough they will start to wobble off course which can result in a complete miss. (Remember misses are bad.)

Barrel twist rates are calculated using the caliber of the projectile and the length of the projectile. Most think in terms of weight of the projectile that determines twist rate but it is really the length. And in general terms the longer the bullet given the same caliber the more it will weigh but the truth is you could have two bullets that weigh the exact same amount but if one has an aluminum core it will be longer than the one with a lead core so the lighter bullet BECAUSE it is longer will require a tighter twist rate than the heavier/shorter one will.

The M16 started off with a 1x14 twist which is enough to shoot 55 grain bullets as long as the temperature of the air was above freezing. If the temp was below freezing the air density was such that the 1x14 twist was too slow and the bullet lost its spin too fast resulting in misses.

So the military changed the rate to 1x12 which solved the problem.

Then comes along the SS109/M855 62 grain round which since it is a longer bullet (because it is heavier) requires a 1x10 to stabilize. Problem was the tracer ammo M856 is even longer than the ball round of the SS109/M855 ammo cause they packed it with enough tracer compound so the bullet would trace all the way out to 800 meters which it almost twice as far as the older tracer ammo that burnt out at around 450 meters. Consequently this newer tracer ammo is a LONG bullet and requires a 1x6 twist rate. So the military compromised on a 1x7 twist rate for the new ammo.

Shorter (which USUALLY means lighter) bullets can be fired in barrels with tighter twist rates than in necessary but longer (which USUALLY means heavier) bullets cannot be shot in barrel twist rates that are slower than what they need. So the most versatile barrel twist rate is one that is tighter as it will shoot all bullet lengths/weights.

An example of what happens when uses a longer/heavier bullet in a too slow twist rate is shooting a SS109/M855 62 grain bullet in a 1x12" twist barrel. What will happen is right around 100 meters the bullet has lost too many RPM's and will actually start flying end over end - with horrible accuracy. The bullet may strike "sideways" - which is called "key-holing" - leaving a sideways imprint of the bullet. Hitting a body with a "key-holing" bullet aint bad - the bad part is the bullet won't go where it was aimed - so you only hit with a "fluke". Since our goal is to hit when we are aiming at something - key-holing is BAD. MIssing is WAY TOO easy with bullets that fly true - missing is almost garaunteed with bullets that don't fly true.

Bushmaster which was really the first company to start building ARs in a big way looked at the available ammunition back in the 80s when they started and back then 62 grains was heavy for .223s. Well the 62 grain lead projectile only needs a 1x10 twist to stabilize and since some 68 and even 69 grain projectiles were on the horizon they figured what the heck lets have our barrels be 1x9 twist. No one is going to be shooting heavier/longer bullets than 69 grains so 1x9 twist will be fine. Remember back then most bullet weights/lengths were in the 40-55 grain area.

Bushmaster soon became the leader in ARs and when more and more companies came on board they followed the leader and had their twist rates be 1x9 also which UNTIL the heavier bullets came along just a few years ago worked out just fine.

Now advance forward to just a few years ago when the war on terror started in earnest. The 62 grain bullet wasnt cutting the mustard. Due to different manufacturing techniques of bullet manufacturers some lots of the M855 wont fragment they way some other lots of M855 will and we all know that if the bullet doesnt fragment it doesnt work as well as when it does fragment. Also the range at which it will fragment is less than 200 meters. Couple this with the fact that the M855 bullet since it has a lead and steel core can NEVER be as accurate as a bullet that has an all lead core making distant sniper type shots a lot harder.

So SpecOps units started looking for a different bullet. (This is what started the ill fated attempt of the 6.8 SPC.) The military match shooters at the time were dominating the shooting matches using bullets that were 75-77 grains in weight. These bullets would allow our boys to make shots out to 600-700 meters with confidence of a hit cause they are extremely accurate bullets. What the SpecOps boys found out that in addition to being accurate these heavy bullets FRAGMENTED when they hit most water based mediums (bodies). And they fragmented with much more vigor than the 62 or even the 55 grain bullets cause since they were heavier/longer they had more material to fragment with! AND they are fragmenting at ranges far in excess of 200 meters.

These heavier bullets are doing such a good job that the 6.8 has since died on the vine.

So if one is getting a new AR or just a new barrel for SHTF purposes it would be my advice to a 1x7 twist barrel. The 1x7 twist will allow you to shoot ALL 55 grain ammo, 62 grain ammo and the 75 or 77 grain ammo. What you wont be able to shoot is the thin jacketed 40 grain ammo but no one would use that for SHTF purposes anyway! In other words you lose NOTHING by going to the 1x7 twist barrel but you GAIN the versatility of being able to use ANY fighting bullet made for the 5.56 family of firearms. So my question would be WHY LIMIT YOURSELF?? You may not have any of the 75 or 77 grain ammo - but why close the door on EVER using it??

Think in these terms - more and more LE units are moving to these rounds - the whole US military is looking at these rounds (because of the great success the SpecOps community is having with these rounds) - so this ammo is only going to be more prevalent as time goes on. DONT GET A NEW BARREL/UPPER that cant shoot these new rounds or you may live to regret it!

If your barrel has a 1x12" twist - you are limited to 55 grain ammo.
If your barrel has a 1x9" twist - you can shoot either the 55 grain or 62 grain (actually up to 69 grains reliably)
If your barrel has a 1x7" twist - you can shoo the 55 grain, 62 grain, 75 and 77 grain - even all the way up to 80 grains reliably

As far as ammo choices:
First choice BY A LONG MARGIN would be the 75 or 77 grain ammo it fragments beautifully from point blank out to many hundred meters.

Second choice would be M193 or equivalent 55 grain ammo if fragments reliably from point blank to around 150 +/- meters.

DISTANT third choice would be the SS109/M855 62 grain ammo which is not a reliable fragmenting round at any range.

Nice post! Mods, please make this a sticky.