1:7 or 1:9

[jmo2003bu]

Ok...I'm fairly new to the carbine world. What does this mean and what is the difference?? Thanks!

 

[downzero]

faster twists will stabilize longer (and typically, heavier) bullets.

1 in 9 will generally stabilize 69s, and may or may not stabilize 75s.

1 in 7 will shoot everything up to 80s at least.

 

[mms]

side note but related does 1-7 have a negative effect on lighter bullets ??

 

[jblomenberg16]

side note but related does 1-7 have a negative effect on lighter bullets ??

1-7 may not stabilize the lighter bullets correctly.

From a little bit of history perspective (very loose, so don't take as exact quote or exact history):

The M193 (55gr) round was developed for the 1-12 twist 20" M16 bbl.
The 1-7 twist barrel was then picked up for the shorter M4 carbine and the longer M855 and 64 gr tracer (can't remember the designation).

There is a rumor and folk lore that due to red-tape etc. that the 1-9 was actually the better choice and should have been what the military went with for the M4. A 1-9 can reliably shoot both the 55gr and the heavier (longer) 62 and 64 gr projectiles. But, for some reason the 1-7 was adopted.


So, there are some current issues that I've read about that there is some limitations on Ammo for what can be used in the M4 and M16's. M193 ball is emergency use only in the M4 carbines, and M855 is the same in the M16's.


Hopefully mvician or another one of our AR experts can supplement what I said with more facts. But, the moral of the story is that a 1-9 barrel is a great compromise for a civilian carbine, as it will happily shoot most of the military spec 5.56 projectiles, as well as some of the heavier and lighter commercial projectiles available.


Now...how about those 1-8 twist fans?

 

[BIGE7.62]

1/7

Urban legend has it that the 1/7 will spin a light bullet to fast and it will explode, Ive never seen it happen. I belive stoner set up the first AR's with a 1/14 twist .A 1/7 twist has one turn of the rifling in 7 ", a 1/14 is one twist in 14", so that would make the 1/7 twice as fast as a1/14 TTBOMK

 

[downzero]

The M193 round was designed for a 1 in 14 twist barrel. Only after they had performance issues was the M16A1 switched to 1 in 12.

1 in 9 doesn't do anything 1 in 8 or 7 can't do better. I like the option of shooting 75-80 grain bullets.

 

[Boiled Owl]

Bullet Stability weight vs. length

A couple of years ago I was given some Barnes Varmint Grenade bullets, 50 or 55 grain. They have no lead in them and are quite long for their weight. They wouldn't stabilize in his 1/12 twist barrel.
I loaded them up for the AR. They still keyholed the target in my friends 1/9 twist and finally stabilized in the 1/7.
Concluded that twist rate and stability are more of an influence on projectile length then weight.

 

[42769vette]

i believe that a 11-7 twist barrel can over stabolize a lighter bullet causing to the lighter bullets nose to dip down at longer ranges. i not 100% sure on that it just seams i read it somwhere

 

[SemperFiUSMC]

Greenhill equation:

T = 3.5 * V^0.5 * D^2 / L

where:

T = Twist rate
V = 150 for velocity under 2800 fps, 180 if velocity is > 2800 fps
D = bullet diameter (inches)
L = bullet length (inches)

Bullet weight is not relevent, other than generally speaking a heaveier bullet will be longer.

 

[dom1104]

I can tell you this from personal experiance, if you are planning on useing a 22lr kit in your gun, the 1/9 works a lot better from an accuracy standpoint.

 

[Fargo]

It is my understanding that 1-7 is because the new tracer (L-110) is required to remain lit for approx 800 yards. This requires a deeper cavity in the base and consequently a much longer bullet even though it is only a (iirc) 64 grain bullet.

As such, L-110 really like 1-6 twist but the 62 grain green tip, being much shorter likes 1-9. 1-7 is a compromise between the two.

Best,

Joe

 

[44 MAGNUM SHOOTER]

My understanding is 1:7 works best in a shorter barrel - 7" - 10.5". Someone correct me if I'm wrong.

 

[MadCity Hoosier]

i believe that a 11-7 twist barrel can over stabolize a lighter bullet causing to the lighter bullets nose to dip down at longer ranges. i not 100% sure on that it just seams i read it somwhere

You're on the right track. Over-stabilized bullets will fall back to earth with the point/tip still pointing up (same orientation it had when it left the bore).

Not really germane to the conversation at hand, so carry on.

To the OP, I didn't see someone specifically answer the question, so I'll explain it, in case you were looking for a very basic answer. The 1 in 7 (or 9 or 12 or....) means that the bullet will perform one revolution in the number of inches specified by the larger number. So, a 1 in 9 twist barrel makes the bullet spin one complete revolution every 9 inches of travel. A 1 in 7 spins the bullet once every 7 inches. Therefore you can see that the smaller the number, the "tighter" or faster the twist of the rifling will be.

 

[jmo2003bu]

You're on the right track. Over-stabilized bullets will fall back to earth with the point/tip still pointing up (same orientation it had when it left the bore).

Not really germane to the conversation at hand, so carry on.

To the OP, I didn't see someone specifically answer the question, so I'll explain it, in case you were looking for a very basic answer. The 1 in 7 (or 9 or 12 or....) means that the bullet will perform one revolution in the number of inches specified by the larger number. So, a 1 in 9 twist barrel makes the bullet spin one complete revolution every 9 inches of travel. A 1 in 7 spins the bullet once every 7 inches. Therefore you can see that the smaller the number, the "tighter" or faster the twist of the rifling will be.

Thanks! All this information has been EXTREMELY helpful! I'm starting to learn about carbines...specifically the ARs. I may come back with some more questions along the way

 

[Don]

YouTube - Light bullets in a fast twist rate, through an AR15 now you have seen the light bullets fly apart.

 

[dukeboy_318]

I've got a 1:9 twist on my ar. I've shot up to 64 grain match loads with perfect holes at distances up to 300 yds with no major difference between the standard 55 gr loads. For me, I have no desire at this point to shoot heavier than that due to cost. The 75 gr and 80 gr loads can be quite a bit more expensive.

 

[sloughfoot]

Urban legend has it that the 1/7 will spin a light bullet to fast and it will explode, Ive never seen it happen. I belive stoner set up the first AR's with a 1/14 twist .A 1/7 twist has one turn of the rifling in 7 ", a 1/14 is one twist in 14", so that would make the 1/7 twice as fast as a1/14 TTBOMK

I have seen it happen.

A 185 series Stainless Mini 14 with a 1/7 twist would frequently blow up 55 FMJ, Winchester Q3131 ammo. I think because of a rough spot in the rifling. I'm not sure because I had a slam fire that wrecked the rifle and Ruger replaced it with a new one. The replacement rifle has never had a bullet blow up, of any weight or style.

It makes a gray "poof" somewhere downrange when a bullet disentegrates after leaving the muzzle. It is easily seen, even by the operator.

Not normal for sure, something was wrong with that rifle......


EDIT to add; I should have watched Don's video before I posted this. The video says it all.....