Barrel twist?

[stitchclimber]

After holding my new AR for less than a week I'm already thinking about the next, and the next and the next, I'll prolly look into getting or building a bull barrel carbine, a plinkster with a carry handle, an AR-10, and a varmint style AR.

My question is barrel twist rates. my barrel is a 1:9, and I was reading that 1:7 would allow you to shoot heavier bullets. What about a 1:8 twist? If someone could help clarify that'd be great.

[oejeffrey]

I shoot a Kreiger fluted barrel with a 1 in 7 twist. I shoot Sierra 77 Grain Hollow Point Boat Tails. Runs you about one hundred dollars for a box of 500. Yes they are expensive but they are worth the money I.M.H.O.


Jeffrey
Life Time N.R.A. Member

[rfc357]

Anything 1:10 or faster (9, 8, 7) will stabilize any bullet you can load to magazine length.

[Pred]

If your wanting a "true" SHTF AR, get a 1x7 twist barrel, 1x8 would work fine also, although most 1x8's are SS barrels so I don't know exactly what your wanting. Some people don't have any problems using bullets that are heavier then 62gr, but others do. 1x9 is fine for a plinker, as most anything you'll find will either be 55gr or 62 gr. However, like I said, a 1x7 or 1x8, you won't have to worry about it. My next AR is going to be a M4 style upper, chrome lined 1x7 barrel for when the world ends....

[stitchclimber]

gotcha, thanks pred

[TACAV]

Taken from the AR15 forums

This is undoubtedly one of the questions that is asked the most. The answer depends on many things, but here are some rough guidelines for those that don't care about the details:
1/14" - 55gr or less
1/12" - 35gr up to 55-60gr
1/9" - 45gr-75gr, possibly up to 77 if you're lucky
1/8, 1/7 - 45gr - 80+gr

[at4rxj]

Originally Posted by rfc357

Anything 1:10 or faster (9, 8, 7) will stabilize any bullet you can load to magazine length.

That's not always true. It depends on how hot the round is loaded. A 1:9 won't always stabilize a 75gr bullet. Some won't even do a 69gr.

1:8 will almost always do it, but a 1:7 is ideal if you're going to be shooting heavy bullets a lot. 1:7 will let you shoot anything 55gr and over very well, and is what I'd go with if you can.

[ClayMan]

My Colt Match HBAR has a 1 in 7 twist and I've found that my groups do open up a bit with 55 grain rounds. I get the best accuracy with 62 grain and 68 grain rounds.

Although its not a huge issue, unless you want perfect accuracy, the 1 in 7 twist actually spins the lighter 55 grain bullet too fast. But I've read on other posts that others with a 1 in 7 twist don't see any difference with 55 grainers vs. 62 grainers.

ClayMan

[at4rxj]

From talking with the gunsmith that built my 2423wssm, you can't "over stabilize" a bullet. The explanation for your case ClayMan could be several things... the position the heavier bullet has in relation to the chamber (heavier = longer), the velocity at which it's going, etc. Different barrels shoot different bullets better than the next barrel, and it's not always just a function of twist rate.

I have heard of this as an issue though with hunting rounds. I've heard of claiming that the faster twist rate on the lighter bullets makes it spin so fast that the jacket starts to separate from the core. I don't know if this is true, but I suppose it could be.

Regardless, for Stichclimber, the best bet may be to stick with a 1:8. I don't think you'll see much difference personally, but do what you think. I'd probably just see what you can find the better deal on. If they're the same, flip a coin... (just make sure you get the right "flip rate" on the coin, or it could throw off the results lol)

[kygunsmith]

"They's a formula out there for the proper twist/vel/weight" , but like the other gunsmith I figure it would be hard to overstabilize.

[USMC]

I prefer 1x7, 1x8, 1x9 in that order. In reality most people shoot 55-62gr ammo, It's cheaper. So it wouldn't matter much which you choose. I happen to shoot a lot of 75-77gr stuff so I like the 1x7 but I do have more 1x9s then anything. Good luck deciding.

[stitchclimber]

So who makes a good 1:7 twist barrel?

or upper?

[USMC]

I've been using Saber Defence barrels lately. They are pretty nice.

PK Firearms | Online Store

[jim433]

cmmg makes them at a fairly reasonable price I hear...I just ordered a complete. It's taking longer than i had hope, but they have a lifetime warrenty on the guns they make...here's link http://cmmginc.secure-mall.com/shop/...=1069317&cat=1&

[Pred]

Check out Del-Ton, they now have their M4 kits/uppers with a DPMS 1x7 barrel option.

[oejeffrey]

Barrels ?? Why take the chance ?? Save up some more money...take your time and buy a Kreiger.
Krieger Barrels, Inc. - Home

I would think instead of asking all of us forum guys if you really want to know about twist rates and what would be proper bullet weight...why not call the barrel makers and ask them ??
Actually Douglas barrels are right down the interstate from me about 20 minutes from my house. Good people......just call them and ask them what they think.


oejeffrey
Lifetime N.R.A. Member

[NRAJOE]

My Colt HBAR is 1:7 twist...I shoot only 62 gr on up.

[ClayMan]

Originally Posted by at4rxj

From talking with the gunsmith that built my 2423wssm, you can't "over stabilize" a bullet. The explanation for your case ClayMan could be several things... the position the heavier bullet has in relation to the chamber (heavier = longer), the velocity at which it's going, etc. Different barrels shoot different bullets better than the next barrel, and it's not always just a function of twist rate.

I have heard of this as an issue though with hunting rounds. I've heard of claiming that the faster twist rate on the lighter bullets makes it spin so fast that the jacket starts to separate from the core. I don't know if this is true, but I suppose it could be.

Regardless, for Stichclimber, the best bet may be to stick with a 1:8. I don't think you'll see much difference personally, but do what you think. I'd probably just see what you can find the better deal on. If they're the same, flip a coin... (just make sure you get the right "flip rate" on the coin, or it could throw off the results lol)

at4rxj, thanks for your explanation. I've always wondered about the "over stabilize" theory. When I've shot 55 grainers, they've always been FMJ rounds (various brands and quality). So I doubt that I was getting jacket separation. But I did get slightly bigger groups vs. 62 grainers, even Wolf bulk rounds (I know that may be hard to believe for some).

ClayMan

[oejeffrey]

Why not call these people ??
Douglas Barrels (304) 776-1341
Kreiger Barrles 262-628-8558
Bushmaster 1-800-998-7928
Compass Lake engineering (850) 579-1208
I am sure there are more sorry about the funny font
Jeffrey Wilson

[RMTactical]

The two most common 5.56 rounds are the 55gr M193 and the 62gr “green tip” M855, both of which are used by the US military. Both are FMJBT designs that are capable of reliable, severe fragmentation when they are moving at speeds over 2700fps. This occurs as the projectile hits soft tissue and the bullet begins to yaw. As this happens, the bullet will often break apart at the cannelure due to the stress and separate creating a devastating wound channel.

Many, including myself believe that the 55gr M193 is a better strictly anti-personnel round than the 62gr M855, especially since it’s lighter weight allows it to fragment out to longer ranges. Also because the fragmentation of the M193 round is more reliable and more violent. When our NATO allies got on board with the 5.56 round, they had been unhappy with the overall penetration abilities of the 55gr M193 in a combat environment, specifically at longer ranges. They introduced the SS109 62gr steel core “penetrator” bullet that would eventually be used in our M855 round.

To the credit of the M855, it’s the better of the two rounds at longer ranges due to its size and ballistic coefficient. It’s not all that bad at shorter ranges either. The steel penetrator also makes it more effective at penetrating hard targets and such.

Keep in mind that 62gr M855 will not necessarily hold the same zero as other types of 62gr ammo. Because of the steel core (steel is lighter than lead), the SS109 bullet is actually longer than a completely lead core 62gr bullet would be. Additionally, the SS109 bullet has a small air cavity in the front of it that increases the length of the bullet even more.

The military first used 1/14 twist barrels and then switched to 1/12 twist barrels to help stabilize the 55gr M193 better in extreme cold temps (the denser air made the 55gr bullet require more twist for stabilization). This goes to show that there are more issues than meet the eye all the time. Air density varies with temperature, humidity, and elevation. Keep this in mind in case you see variations in your groupings in different area and/or conditions.

The most common twist rates for AR-15’s today are 1/7 and 1/9. The military uses the 1/7 twist in their barrels in order to stabilize the very long M856 tracer round that compliments the M855. These rounds are both roughly the same grain, but the 800 meters worth of incendiary tracer compound stored in the M856 round makes it longer than the M855 projectile and in need of a faster twist to properly stabilize it. The 1/7 twist rate was a compromise between the best rate for each round.

The 55gr M193 round stays around in the military because of the small amount of 1/12 twist barrels still out there in the military at the moment. Speculation tells us that the 55gr M193 will inevitably be phased out as the last of the 1/12 barrels are replaced by 1/7 twist barrels.

The biggest drawback to the 1/7 is that you cannot shoot anything much lighter than 55gr in general. Even with 55gr the 1/7 barrels tend to be slightly less accurate than they are with 60gr+ ammo. It shouldn’t be too big of an issue with the already superbly accurate AR-15 though unless you are a target shooter or a designated marksman stuck with 55gr M193. 1/7 twist barrels wear a little faster than slower twists as well.

The military never used 1/9 twist barrels although it’s probably the most popular civilian AR twist rate.

The 1/9 shoots the 55gr M193 very accurately though and it can handle lighter loads like 45gr varmint/frangible bullets just fine. It is generally accepted by many that 1/9 twist barrels will only reliably stabilize bullets up to 69gr. Despite this, many folks with 1/9 twist barrels still shoot the newer 77gr just fine. Sierra claims that 1/9 twist barrels will stabilize their 77gr Sierra MatchKing BTHP without issues. Bushmasters website also says, “you can safely and accurately fire up to a 75 grain bullet in the Bushmaster chrome lined 1 in 9" twist barrel - even an 80 grain can be fired safely but accuracy will suffer.”

I will concede that 77gr is probably the very end of the spectrum as to the weight that a 1/9 twist barrel can stabilize sufficiently. Once again, temperature, humidity, and elevation will play a role in this case.

Most AR-15 barrels out there (chrome moly) are button rifled and the twists are not usually exactly 1/7 or 1/9. Any given 1/9 twist barrel may truly be a 1/8.5 twist barrel or a 1/9.5 twist barrel. Do some testing and find out for yourself what weight/length bullets your barrel can and cannot handle.

Admittedly, the trends seem to be moving towards heavier bullets in this caliber for combat use and if you plan on shooting higher grain bullets, in general, the 1/7 is probably the best twist rate for you. Hopefully you can afford to shoot these expensive rounds.

For many of us, 55gr bullets (XM193) are the most affordable. It allows us to train and shoot more. They yield acceptable results for self defense as well.

Truthfully, I cannot understand why the 1/8 twist is not more common than it is. As it is right now, it’s not a very common twist, at least when compared to the 1/9 and 1/7.

I have 1/9 twist barrels and 1/7 twist barrels. I just can’t make up my mind on this issue so that’s why a 1/8 would probably solve my dilemma and is possibly the best compromise. I want the 1/9 twist barrels so I can shoot 55gr M193 as accurate as possible, as that is my primary choice of ammo. On the other hand, I have the faster twist rate to more accurately shoot the 68gr Black Hills BTHP that I find not only affordable (relatively speaking) but it is very effective and has a fragmentation threshold of 2400fps (nearly 200 yards of reliable fragmentation range from a 20” barrel).