308 Twist? 1:12 or 1:10

[Olympus]

The two guns are the Rem SPS Tactical and the AAC Tactical. I know the AAC barrel was designed to shoot heavy bullets subsonic for use with a suppressor. Both are 20" barrels.

For me and my use, I would not be shooting loads over 175gr and I would not be using a suppressor. So for about $60 more, is the AAC model still the better choice with its 1:10 twist even though I'll likely only be shooting 168 to 175gr bullets?

 

[Art Eatman]

All I know in twists for 30-caliber is that 1:10 works very well for bullet weights from 110-grain through 180-grain.

My offhand opinion is that 1:12 might be too slow for best accuracy with 175s, but that's just an opinion.

 

[Nature Boy]

I have an FN SPR that has 1:12 twist and shoots 175g and 180g the most accurately. It's got a 26 in barrel so maybe that's a factor. I'm curious to know what feedback you get

 

[lightman]

I built a 308 hunting rifle with a 1-12 twist and I would go 1-10 if I did it again. A lot of match rifles are 1-11 or 1-11.25. The 1-10 just works so well for so many different bullets.

 

[tark]

The 1-10 twist in a 30 cal trend was started by the US army way back in 1892 when we switched from the trapdoor to the Krag. It was discovered that a 1-10 twist was needed to stabilize the long 220 gr. bullets the army was adopting. Later, when we went from 220 down to 174 and finally down to 150 grain bullets it was discovered the 1-10 was faster than needed but still gave good accuracy....so it was retained. It has been standard ever since.

For bullets under 200 grains, the 1-12 is may be a better choice, since it won't over stabilize the lighter weights. A 150 grain bullet at 2900 out of a 1-10 twist is going to be doing the horizontal hula for the first 100 yards or so. The point of the bullet will stay true, but the rear end will be dancing around a bit. In the same way a 55 gr bullet is over stabilized when shot out of a 1-7 twist AR.

Having said all that, in the real world, I doubt the average person will ever know the difference when it comes to performance on either game or paper. The 1-10 twist can do it all, the 1-12 can do most of it....

Can't go wrong with the 1-10..

 

[Newtosavage]

My Savage 11 is 1:10 and I'm getting great groups out of it to 300 yards with both 125 and 150-grain bullets. Two days ago, I shot a 1" group at 300 with 125 grain Noslers. No accustock and thin tapered barrel.

 

[Olympus]

I doubt I would be shooting 150gr bullets. More likely 168 to 175gr. This would be for a long range build, but not competition.

 

[lysanderxiii]

A few nits to pick...

The Army ".30-06" Ball ammunition went from the 220 gr M1903 to the 150 gr M1906 to the 174gr M1 and back to 152 gr M2. The M1 Garand was designed to use the 174 gr M1, or the 152 gr M2, so the 1-10 twist.

The M14 was only designed to use 147 gr M59/M80 Ball so it had a 1-12 twist.

A 1-7 twist will not "over-stabilize" a 55 gr M193 projectile.

And, the base of the bullet (nearer the CG) will remain more or less in place while the nose yaws.

EDIT: a 1-12 twist will stabilize any .308 bullet (of lead and copper) up to at least 180 gr at muzzle velocities over 2550 fps under standard conditions, however, this may not be the case in cold denser air, which is why the Army adopted 1-10 twists for match rifles using heavy bullets.

The 1-10 twist in a 30 cal trend was started by the US army way back in 1892 when we switched from the trapdoor to the Krag. It was discovered that a 1-10 twist was needed to stabilize the long 220 gr. bullets the army was adopting. Later, when we went from 220 down to 174 and finally down to 150 grain bullets it was discovered the 1-10 was faster than needed but still gave good accuracy....so it was retained. It has been standard ever since.

For bullets under 200 grains, the 1-12 is may be a better choice, since it won't over stabilize the lighter weights. A 150 grain bullet at 2900 out of a 1-10 twist is going to be doing the horizontal hula for the first 100 yards or so. The point of the bullet will stay true, but the rear end will be dancing around a bit. In the same way a 55 gr bullet is over stabilized when shot out of a 1-7 twist AR.

Having said all that, in the real world, I doubt the average person will ever know the difference when it comes to performance on either game or paper. The 1-10 twist can do it all, the 1-12 can do most of it....

Can't go wrong with the 1-10..

 

[Olympus]

So I still don't know which twist to go with.

 

[Nature Boy]

How does barrel length play into this?

 

[Archie]

From the files of the old man, "A lot of overspin is better than a little underspin". Get the faster twist.

No. Barrel length has nothing to do with it.

 

[The Big D]

There is no downside to 1:10 twist.

 

[Minnesota Wild]

OK, barrel length does not play into this. Barrel length will give you velocity, but twist rate is about stabilizing the bullet, nothing else. I don't know how much you know about why some barrels have faster twist rates for the same caliber, but your question makes me think that a short explanation would be helpful.

Bullets become unstable as they get longer, i.e. heavier at the same bore dimension. That is because to make a bullet heavier at the same diameter it must be longer. In order to gyroscopically stabilize a longer object you need to provide more gyroscopic stability, i.e. make them rotate faster. This is why a baseball is easy to throw accurately, but a quarterback needs to work hard to provide spiral to a football in order to throw it well. This effect becomes more pronounced at longer ranges as the bullet begins to become unstable and thus the impact of the bullet becomes less consistent.

To say that their is no downside to a 10 twist is mostly accurate. There is some discussion about whether a faster twist slows velocity and overstabilization can cause some other problems not worth discussing...if a 10 twist had 0 downside it's what all manufacturers would set as a standard.

So to answer your question, a 10 twist in 308 is popular for long range shooters so they can shoot heavy bullets without going subsonic before 1000 yards, as the buffeting of the transonic shift will also destabilize bullets and make them less accurate. Thus, if you plan to shoot 1000 yards a 10 twist is a good choice so you can shoot heavier bullets. 12 twist is a good choice for 168 or under and is still the rate that some producers (Remington) put on their "police" model guns. 11.25 is the rate that the military found to be ideal for 175 grain SMKs in their 118LR round and so that is what many of the 5r barrels are cut for.

in other words, it just has to do with your intended range and bullet weight. If you want to be able to shoot very heavy bullets at very long ranges, got with a 10 twist. I would not hesitate to buy a Remington 700p (as I have not) with a 12 twist as they are very, very accurate to over 800 yards.

 

[Water-Man]

Generally, a 12 twist is recommended up to 170gr. and a 10 twist for up to 220gr.

 

[joneb]

I would opt for a 1:10 for a 20" barrel, a 1:12 for 24" just my 2 cents.
Longer barrel will give more velocity which equals more RPM's to the projectile.
So with out doing the math, a 175gr bullet from a 24" 1:12 twist would be spinning about the same as from a 20" 1:10.
http://www.bergerbullets.com/twist-rate-calculator/

 

[Nature Boy]

Thank you joneb, that's exactly what I was thinking.

 

[Catpop]

Jmtcw, longer barrels allow for more pressure build with slower powders producing more velocity.

 

[Bart B.]

When the .308 Win was first used in competition during its first three years, folks getting the best scores, winning matches and breaking all the records held by the .30-06, several facts emerged regarding bullet twists and bullet weights used.

The 22" barrel in M14's shot 172-gr arsenal match bullets as well as 150, 168 and 180 grain Sierra match bullets with excellent accuracy through 600 yards. Only the Sierra 180's did well out to 1000. The others went subsonic before touching paper. When rebarreled with 1:11 or 1:10 twists, nothing changed with good bullets. Lesser quality bullets that were more unbalanced started to shoot bigger groups; centrifugal forces made them jump more off the bore axis upon exit.

24" barrels in 7.62 converted Garands with 1:12 twist barrels shot those same bullet weights as well as Sierra 190's very accurate through 600 yards. At 1000 yards, only the 150's fell short as they sometimes didn't stay supersonic. Same results with 24" 1;12 twist bolt guns. 1:11 and 1:10 twist Garand barrels did as well, but again, poor bullets in 1:10 twists didn't do so good.

26" bolt gun barrels with 1:11 twists seemed to do best overall. With reduced loads for 150 to 168 grain bullets, accuracy through 300 yards was phenomenal with 150 to 168 grain bullets. At longer ranges, only the 150 sometimes fell short muzzle velocity wise. 168 through 210 grain bullets did very well through 1000 yards with 1:11 twists.

When the British Empire switched from their .303 to the 7.62 NATO round for their fullbore long range matches, they learned the arsenal 147-gr. bullet (what their rules required) was not leaving fast enough to stay supersonic through 1000 yards but did well through 600 yards. So they started using 30" barrels to get another 150 fps of velocity and with a 1:13 twist, all was perfect for best long range accuracy.

Winchester introduced their .308 Win round with 1:12 twist barrels and sold ammo with bullet weights from 150 to 200 grains. They all shot very accurate through 300 to 400 yards for a hunting rifle. All their match grade Model 70's had 26 inch long 1:12 twist barrels and shot WCC 197 and 200 grain bullets very accurate through 1000 yards.

Each bullet needs a certain rpm rate to stabilize it in flight to the target. Whatever combination of twist and muzzle velocity that does it is good. Too fast a spin starts degrading accuracy as all bullets are not perfectly balanced. RPM equals muzzle velocity in fps times 720 divided by twist in inches.

 

[The Big D]

When rebarreled with 1:11 or 1:10 twists, nothing changed with good bullets. Lesser quality bullets that were more unbalanced started to shoot bigger groups; centrifugal forces made them jump more off the bore axis upon exit.

You'ev got the effect right, but the description of the physics a bit messed up.

The problem that arises from a faster twist is that when a bullet leaves the muzzle, it ceases to rotate around center of shape, and starts to rotate around center of mass. This results in a one time "jump" in a random direction away from the bore axis. The direction is random, because in a bullet where COS and COM are not the same it's random which side will be the heavy side when it leaves the muzzle.

The size of this effect is dependent on two things: the difference between COM and COS of the bullet, and the twist rate. If you have a well made bullet (and nearly everything is, these days) you get essentially no jump even at fast twists. There have been more than enough 1/4MOA groups shot with 1:8 and even 1:7 twist barrels to think that this effect means anything to the match bullet shooter. If you're shooting Bob's Mystery Maybe-Plated Bullets (TM), then you might have an issue.

The basic fact that OP needs to know is that there is no negative accuracy impact with a 1:10 twist barrel under any circumstances with any normal .308 projectiles. It is simply the superior solution. The same can't be said for 1:12 - in a cold temperature/low altitude/heavy projectile combination it will be marginally stable and you will see increased drag due to bullet wobble.

 

[Bart B.]

I don't think I messed up anything. Just explained it differently. I understand your point.

And even the best bullets have some level of unbalance for some in a given lot. Otherwise, the most accurate 100 yard benchrest rifles would shoot much smaller at worst than about 3/10ths MOA which is where the records are. They're 4/10ths at 200 and near 5/10ths at 300.

Tests have shown the best bullets leaving at the same muzzle velocity have about a 1% spread in BC as their time of flight over the first several yards is not the same. Their small unbalance causes that.

What's a normal 30 caliber projectile?

A friend took several hundred of the best match bullets made, spun them in a collet plugged into a Dremel tool spinning 30,000 rpm. An amp meter was in series with it and when more current was drawn indicating the bullets were unbalanced more current drawn was higher. Some flew out of the collet bouncing off the walls in the shop. Perfect balanced ones drew the least and constant current then set aside for testing. He shot several 10-shot test groups at 600 yards; largest was about 1.5 inches with the perfect ones. The unbalanced ones shot up to 8 inches.

 

[Olympus]

Well I decided up go with the AAC model with the 1:10 twist. Hopefully it will work out. Got some 168gr HPBT SMK bullets to try first with some 4895 powder.

 

[tark]

LysanderXIII I am confused. (Happens a lot) Did I mis-understand or did you state that on over-stabilized bullet will experience a yawing at the nose but the base will remain true in flight?

How can this happen? It would seem that the instant that the nose of the bullet deviates from its flight path that the bullet would take off in that direction. I can understand how the bullet could fly true if the nose of the bullet stays pointed at the target and the rear end dances around a little, but.... I can't understand how the opposite could result in a bullet flying true toward the target.

I await further education.

 

[redbullitt]

I like the 10 twist. I don't think you will have any regrets. You may change your mind in the future and play with other bullets!

The 208 AMAX actually is pretty interesting with proper load development and powder choice. FWIW my 10 twist shoots 165 cast bullets 2-3 inches @ 100 yards. May not sound like much to the jacketed folks, but I was pretty happy considering I didn't really even do much load work lol. It will shoot sub MOA with 178 AMAX, 168 or 175 smk, and just about any similar bullet. Even better, it does well with subsonic 180 RN and similar bullets. Cant ask for much else really as an all around rifle.

 

[lysanderxiii]

LysanderXIII I am confused. (Happens a lot) Did I mis-understand or did you state that on over-stabilized bullet will experience a yawing at the nose but the base will remain true in flight?

How can this happen? It would seem that the instant that the nose of the bullet deviates from its flight path that the bullet would take off in that direction. I can understand how the bullet could fly true if the nose of the bullet stays pointed at the target and the rear end dances around a little, but.... I can't understand how the opposite could result in a bullet flying true toward the target.

I await further education.

In that post each sentence was its own paragraph independent of the others.
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The 4th paragraph was in reference to your statement that the nose follows the trajectory and the base wobbles. The opposite is true for all bullets.

Any object's trajectory will be defined by the path of the CG, and any rotation or tumbling motion will be centered on the CG. A tumbling bullet's CG will still follow an arcing trajectory just like a stabilized bullet, just much abbreviated, due to the increased drag.

The nose of the bullet produces lift, this lift creates an 'overturning moment' that tries to flip the nose over and have the bullet fly base first. If the bullet where not spun, this is exactly what would happen*. However, with the bullet spinning, there are gyroscopic forces that tend to force the nose back in line with the flight path, if the gyroscopic stability factor (SG) is 1.00 or greater.

The CG of a spitzer bullet is nearer the base than the nose, since all rotation, both yaw and roll are about the CG, the base moves very little and the nose yaws in a helix through the air.

This is the path of the nose of a bullet (specifically, a M193 Ball), the path of the CG is a straight line through the center of the box.


(The M193 bullet from a 1-12 twist has a neutral dynamic stability, note that the large precessions do not damp out.)
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*Then as the base heads to the front you get all sorts of turbulence and the rotation become unpredictable.
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Next subject: over stabilization. This is an over stabilized bullet:

It is normally only a problem at high angles of departure (howitzers and the like). You can calculate the rpm necessary for this, and for a .75" long .224" diameter bullet, 1-7 twist will not do it. And even if you did "over-stabilize" a rifle bullet, the angles of departure are usually low enough that it will not cause problems.

 

[Frostbite]

Thank you for this ^^^!