AR barrel twist vs bullet weight

[D.B. Cooper]

So I'm new to the AR world (some may recall my earlier first post about lower receivers), and I have a question/am in need of a memory refresh on barrel twists and bullet weights.

As I recall from Basic Training, there were two barrel twists in these rifles: a 1:7 and a 1:9. (I forget which version had which twist rate. Also as I recall, one twist rate would fire the 55 grain bullet well, but not the heavier 62 grain-it would tumble pretty quickly after leaving the barrel, but the other would fire both pretty well. Does any of this sound familiar?

So I have my lower, and I'm getting a A2 upper with a fixed handle, which 20" barrel (20" is the right length for a regulation USGI rifle, isn't it?) should I get to have the most versatility? (Plus I guess I should be wanting to shoot the cheaper 55 grin ammo.)

Any reason to buy one over the other?

Thanks in advance.

 

[C-grunt]

1 in 7 is military twist. Will stabilize rounds up to around 80 grains.

1 in 9 twist will stabilize rounds up to about 70 grains.

Both of those statements are very generalized though and each barrel is different.

Personally I would go 1 in 7.

 

[C-grunt]

Either will shoot 55 and 62 grain just fine.

 

[Tinpig]

And my 1970s Colt SP1 has a 1 in 12 twist. Very accurate with 55 gr.

Tinpig

 

[LoonWulf]

It depends on what bullet weights you want to shoot, if you nver plan on shooting heavy stuff a 1-8 or 9 is plenty. I tend to favor that twist rate because everything i shoot is short enough to stabilize well. For general target and smaller med game...goats sheep i shoot 65grn sierra btsp, for anything over 100-120lbs I like speer 70 semi spitzers. I have some random 80grn softpoints thqt stabilize in my 1-9mini, but 75+ target bullets do not.

 

[ironworkerwill]

Twist has little to do with weigh and more to do with length. A heavy soup can style projectile takes less twist to stabilize than a light long VLD style projectile.

 

[Water-Man]

How do you plan on using your rifle?

 

[JO JO]

My 1/9 twist shoot the 69gr very well

 

[Varminterror]

Twist has little to do with weigh and more to do with length.

Not entirely accurate. Longer bullets do need more spin to stabilize, but weight - or rather Specific Gravity (commonly approximated as Sectional Density) - does matter, and matter a great deal.

Anything from 1:7" clear up to 1:12" or even 1:14" will typically stabilize up to 55grn bullets in 20" or longer barrels. In short carbine barrels, 1:9" will stabilize up to between 62 and 70grns before things tend to get wonky - at least out to 200-300yrds. 1:8" barrels will stabilize up to 77-80grn, and 1:7" is typically needed for supreme precision and stability with the 80-90grn pills, but the guys shooting these bullets tend to know exactly what they're getting into, with bore-rider throats and single loading over-mag-length cartridges to reach the black on the MR1 at 600yrds.

I shoot the 69 and 77grn SMK's and the 75 Amax (single load) out of 1:9" 20, 24, and 26" AR's regularly. I shoot the 69's out of one of my Shilen tubed 1:9" 16" M4 Hbar clone - no keyholing and sub-moa at 400yrds off of the bench with this one.

 

[ironworkerwill]

Sure, lots of things contribute to stability. Elevation, velocity, and plastic tip length are also factors.

The most important factor in determining stability is projectile length.

 

[climbnjump]

Okey dokey then... You can go here for a good twist rate stability calculator on the Berger Bullets web site. While the calculator only includes preloaded data for Berger bullets, you can edit the inputs to put in data for any other bullet that you have data for.

 

[carbine85]

1:9 works very well. The 1:8 is the best overall twist for the AR. 1:7 isn't needed. It was adopted by the military for tracer rounds. Everyone wants what the Army has which is why so many get caught up in the 1:7.
Buy the twist that matches best to what you will be shooting the most.

 

[Varminterror]

How do you plan on using your rifle?

Buy the twist that matches best to what you will be shooting the most.

This is really what matters the most. I run 1:9" tubes for my coyote calling rifles, as I know I'll be running 50, 53, or 55grn V-max's with the occasional hog hunting trip using 60grn Partitions, only shooting out to ~400yrds. No need for the extra spin of a 1:7" tube for 50-60grn pills, even in short barrels. For a match rifle, I run 1:7" or 1:7.7" with longer barrels to make sure I can stabilize 80 & 90grn Bergers. For guys wanting to blast zombies at short range without worrying too much about precision, any twist with any bullet will be fine.

Just gotta match up your rifle to your expectations.

 

[JohnKSa]

Here's a fairly thorough treatment of 223/5.56 rifling twist issues.

http://thefiringline.com/forums/showthread.php?t=530927

 

[D.B. Cooper]

Thanks for all of the comments.
How will I be using the rifle? Probably not that much. Pretty much entirely at the range. Possibly Service Rifle competition, but I'm not a serious competitive shooter, so I'm not going to buy NM parts at NM prices.

I'm mostly doing this now because the world is going to change on Nov. 9.

So primarily, it's just a target rifle.

I kind of decided that if I was going to own an AR it was going to be identical to my Basic Training rifle, and I'm finding that problematic. No one has an A2 (fixed handle) upper with a 1:7 twist; they're all 1:9 or 1:8.

From what I've read, I'm pretty late to the game and true A2 parts have mostly gone away.

 

[hdwhit]

Here are some articles about pairing barrel twist rates and bullets.

• http://www.gunsandammo.com/ammo/pair-barrel-twist-rates-ammo/
• http://www.nssfblog.com/firstshotsnews/the-truth-behind-twist-rates/
• http://info.stagarms.com/blog/bid/371861/How-Barrel-Twist-Rate-Affects-Ammunition-Choice

A couple of my own observations:

1. Above about 60/62 grains, velocity in the .223 Remington declines disproportionately to the increase in weight. Since energy is a function of the velocity squared, this means that even with the heavier bullet retaining more of its velocity downrange, the heavier bullet may nevertheless arrive on target with less energy than a lighter bullet and this can impact terminal performance.
2. A faster twist rate subjects the rifling to greater stresses as the rifling must impart a more rapid spin (i.e. greater angular velocity) to the bullet. While I am not aware of any published scientific studies of barrel life for various twist rates (although there is an abundance of conflicting anectdotal evidence and "folklore"), it stands to reason that the additional work being performed on the bullet by the faster twist rate, which contributes to faster twist rates generating higher pressure and heat, will - all other independent variables being constant - wear more quickly than a slower twist rate.

Thus, I would suggest you get the slowest twist rate that will stabilize the bullets you are going shoot through the rifle. In my case, I don't shoot anything heavier than 60 grain nor do I shoot beyond about 200 meters, so I got a 1:9 barrel.

 

[C-grunt]

A good barrel will last a long long time if you don't abuse it. My friend has a BCM 11.5 inch gun with around 20k rounds through it. Still accurate enough to hit a silhouette target at 500 yards.

 

[D.B. Cooper]

Thanks much for the article links; they helped a lot. From what I gathered, both the 1:7 and the 1:9 will shoot either of the cheap/bulk ammo (55 and 62 grin) just fine. As one of those articles stated: "If you want a mil-spec clone, use 1:7" (Which is where I wanted to go with this.)

20" upper halves with fixed carry handles AND in 1:7 twist are as rare as hen's teeth. (I actually haven't found any at all anywhere online.) Closest thing I've found is a Del-Ton complete upper half with 1:9 twist on about an 8 week back order.

The other thing I've found is a Green Mountain Barrels 20" barrel in 1:7. What do you guys know about that brand? Are they top, mid, or low tier? (At $174, I"m guessing lower or mid.) Is this a "run away from" brand?

I'm thinking I can match this to a Rock River Arms upper. Of course, that will require the additional cost and wit of taking it to a gunsmith for headspacing-almost wonder if it's worth it.

I would much prefer to buy a factory completed upper.

 

[D.B. Cooper]

Those barrels aren't pinned for a front sight post, so I don't know how to attach the front sight to it.

 

[LoonWulf]

Green mountain makea pretty decent barrels, id put them middle of the pack interms of quality.

Building your own uppper isnt hard. since your looking for something specific, that you cant buy as a complete set, id really suggest building your own.

 

[Coal Dragger]

Bravo Company Machine has 20" rifle barrels in stock right now. 5.56×45 NATO chamber, 1:7" twist. Mil spec CMV 4150 steel, chrome lined bore and chamber. Phosphate finished. Barrel extension does feature M4 feed ramps though. These should be an excellent long lasting barrel.

Not fond of the profile myself, but to each his own.

 

[Creature]

 

[Creature]

Personally, I prefer a 1:8 with a Wylde chamber.

 

[Crowcifier666]

Building my first AR as well. Is the Wylde chambering more of a benefit than deterrent when shooting bulk ammo? I also plan to load for this rifle, but I can't keep up on a single stage....

 

[Coal Dragger]

Some feel the Wylde chamber is a benefit over the longer lead of the 5.56mm NATO chamber. All other factors being equal in the rest of the barrel, the increase in precision potential of the barrel that a Wylde chamber will give is pretty minimal for anyone other than a competitive shooter firing match grade ammo.

I personally have only 5.56mm NATO chambers in my 3 AR's. One is a 16" Colt M4A1 with the new gov't medium contour SOCOM profile barrel, and the other two are both Daniel Defense 18" S2W medium/heavy contour barrels.

The Colt is button rifled and chrome lined, the two DD's are both cold hammer forged and salt bath nitrocarburized. All are CMV 4150 steel with 1:7" twist.

All of them are capable of shooting sub MOA 10 shot groups at 100 yards with carefully assembled match ammo using 69gr or 77gr SMK's, and right around 1 MOA shooting 55gr Nosler BT's, which I can probably dial in a bit more if I have time. Keep in mind these are 10 shot groups, using less strict 5 shot groups I've seen .5 MOA on occasion. Using a larger data set of a 10 shot group gives me confidence in stating that a good quality barrel with a 5.56mm NATO chamber is capable of very good precision particularly with longer heavier bullets 60gr and up.

All while maintaining the known high degree of reliability in a wide variety of conditions that the NATO chamber spec provides for. Could this precision be improved upon? Sure, but a tighter, less forgiving chamber and lead will be required. Were this a barrel for a precision bolt action rifle, or a dedicated match rifle I'd say go for it! For a general purpose carbine that you might have to stake your life on, that needs to function 100%? Nope, stick with the NATO chamber it's still capable of great accuracy, and makes for a more reliable weapon.