Barrel Twist

[Hammered Squirrel]

So Ive always wondered if the barrel twist is a full twist in that many inches or that many twists in and inch? Ive never really paid that much attention to it, being that up until recently I pretty much only shot shotguns. I guess what I'm asking is if a 1-7 twist is better then a 1-12 twist? How heavy of bullets...most likely 50 to 62 grains. Just planning ahead for when I put together a dedicated 'yote AR.

Thanks

[Vlad]

Let’s start with what caliber? I assume you are thinking .222/5.56. That being said, the faster the twist (higher number) the lighter bullet it will stabilize. A 1/12 is for “lighter” (<55) bullets, a 1/7 for “heavier” (>62) bullets. This of course is totally a generalization. I have 1 in 9 twists in all my .223s and have been getting good results with 50 Varmint Grenades and 69 Gr HPBTs. It all boils down to what you are looking to do…

[carver952]

A 1/9 twist means that the bullet will make a complete revolution for each 9 inches down the barrel
1/9 is the best overall for magazine fed 55-73 grain out to 600yards,
1/12 is good for varmint 40-50 grain,
1/7 for 75 grain or heavier for long range 800 yards bolt action single fed if length >2.26"

[xd ED]

A 1/9 twist means that the bullet will make a complete revolution for each 9 inches down the barrel
1/9 is the best overall for magazine fed 55-73 grain out to 600yards,
1/12 is good for varmint 40-50 grain,
1/7 for 75 grain or heavier for long range 800 yards bolt action single fed if length >2.26"

I have to ask:
Just because the barrel is, per your example a 1/9", meaning the rifling makes 1 revolution in 9", does that mean the bullet spins at the same ratio?
I know many seem to assume that, but is that a certainty?

[jshuberg]

A barrel with a 1:7 twist is one full twist of the rifling in 7 inches of barrel.

As far as which twist rate is best, there's a long and short answer. Short answer - for 50 to 62 grains a 1:9 will work fine. Anything with a faster/tighter twist and you risk over stabilizing the lighter bullet. Some shorter .223 varmints bullets will even lose their jackets and disintegrate in the air with a 1:7 twist. The original AR-15 had a 1:14 twist, but most barrels these days will have 1:7-1:9.

Long answer is all about gyroscopic stabilization. When a bullet leaves the barrel it is traveling extremely fast, and slight variances in atmospheric conditions between the muzzle and the target will impart angular forces on the bullet, which will cause a change of orientation as it travels, and it will take a different and unpredictable path to the target. This results in extremely poor accuracy. This is where the need for gyroscopic stabilization comes into play.

When a bullet is spinning at a very fast rate, any angular forces imparted on it will be resisted, and it will attempt to remain oriented in the same direction. Even if the bullet weren't moving through the air, but just spinning in space, if you tried to push the nose of the bullet to point in a different direction, it will resist you. A spinning top works the same way, that while it's spinning it will resist the force of gravity and remain perfectly upright. Until it starts to slow down.

When a bullet is spinning fast enough to resist the atmospheric conditions it travels through, and it remains oriented with its arc of flight, it is stabilized. When the atmospheric conditions are enough to overcome this resistance and change the orientation of the bullet during flight, it is under stabilized. If the bullet is spinning so fast that the nose of the bullet doesn't follow the arc of its flight as it drops, but rather stays oriented with the barrel of the rifle, it is considered over stabilized.

The bullet resists a change in orientation by using its rotational energy to oppose the external forces being applied - and the spin rate slows down. The greater the external forces applied to the bullet, the more rotational energy is used to oppose those forces, and it slows down even more. A bullet that is minimally stabilized in calm weather may become under stabilized in windy/choppy conditions partially through it's flight path, as it may use up enough of its rotational energy to become effected by external forces as it slows. The farther the target distance, the longer the bullet is in the air, and the more external forces are applied to the bullet. A bullet that is fully stabilized to a 100 yard target may become under stabilized in the same atmospheric conditions out to a 1000 yard target.

Most people think that the biggest factor in choosing a twist rate for a particular bullet at a particular velocity is bullet weight, which isn't correct. Its actually bullet length, but it just so happens that heavier bullets also tend to be longer. The reason is leverage - the longer the bullet the easier it is for the same external force at the tip of the bullet to push it off it's orientation, and the more rotational energy is required to resist that force. Heavier bullets do have the advantage that they have slightly more energy than lighter bullets that are spinning at the same RPM.

There are a number of different formulas and calculators for determining the ideal spin rate for a given bullet. Here is a decent calculator and a brief history of some of the equations used to predict stability:
http://kwk.us/twist.html

Note that bullet RPM = muzzle velocity X 720/twist rate

[Hammered Squirrel]

Well ok... I guess since I shoot mostly 50 to 55 grain stuff that I'm more leaning towards a 1:7 twist rate. Now would a 1:8 be better? Or would I not notice a change at all? Also what upper do you guys like for bull barrels? Ive been looking at a white oak and I was also looking if anyone could maybe give me a review of one of those SOTA Arms uppers?

[jshuberg]

Well ok... I guess since I shoot mostly 50 to 55 grain stuff that I'm more leaning towards a 1:7 twist rate.

I'd suggest a 1:9 unless you are building something with a barrel length under 14"

[45Badger]

Well ok... I guess since I shoot mostly 50 to 55 grain stuff that I'm more leaning towards a 1:7 twist rate. Now would a 1:8 be better? Or would I not notice a change at all? Also what upper do you guys like for bull barrels? Ive been looking at a white oak and I was also looking if anyone could maybe give me a review of one of those SOTA Arms uppers?

What is your intended use? What range/distance? 100 yards and paper targets are not difficult. 300 yards and a small critter need a very different solution. 600 yards is a whole new game.

[Hammered Squirrel]

What is your intended use? What range/distance? 100 yards and paper targets are not difficult. 300 yards and a small critter need a very different solution. 600 yards is a whole new game.

Dedicated coyote rifle. Fixed stock and Im deciding between a 18 in or a 20 in bull barrel free float upper. so Im looking at 300 yards maybe a little bit more and a small to medium critter. Maybe even out to 600 if I can even try.

[45Badger]

A good 1in 8" twist bull barrel should give you a laser with plenty of punch to cleanly drop coyotes or explode pdogs at you distances.

[Hammered Squirrel]

A good 1in 8" twist bull barrel should give you a laser with plenty of punch to cleanly drop coyotes or explode pdogs at you distances.

Ok, Ive seen some 1in 8 that I like. What grain bullets would you recommend? Would say 50 to 55 grain HP or FMJ be good? I know in this state you cant shoot any critter with FMJ's.

[arizona98tj]

I wouldn't use FMJ for varmint hunting for the same reasons I wouldn't use FMJ for deer hunting.
As for the bullet weight within the range you mentioned, I would use the the that gave me the best accuracy. Not all rifles like all bullets.

[45Badger]

Ok, Ive seen some 1in 8 that I like. What grain bullets would you recommend? Would say 50 to 55 grain HP or FMJ be good? I know in this state you cant shoot any critter with FMJ's.

My 1/8 shoots factory 55 grain VMax about .75" at 100 yards. It shots the heavier Hornady stuff (75 gr?) into a quarter inch when I do my part. The reason for "heavier" is the ability to stabilize a longer projectile (as noted above) for longer ranges. At 100 yards, almost anything will do. At 300 to 600, you're likely to get much better accuracy and bullet performance on live game with a heavier bullet.

[Hammered Squirrel]

I never really planned on using FMJ for hunting, but awhile back while yote hunting we were informed that you can't use FMJ for hunting. It's just that it's so readily available and relatively inexpensive versus other bullet types. Also I don't reload( YET), so everything I shoot is going to be factory new. My plan is a 18 or 20 inch free float quad bull upper with a poly lower with a fixed stock. A good piece of glass on top and a bipod.