1 in 7/9"?

[daveyburt]

Hi Folks,

I've been reading up on some .233 rifles and have noticed folks discussing barrel twist numbers; 1 in 7", 1 in 9", etc....

I'm assuming that means one bullet spin in 7" of barrel travel type of thing?

Please correct me if I'm wrong and, if you could, let me know what the benefits are of each variant.

Thanks!

 

[daveyburt]

....i think i just figured it out....but please verify.

1 in 7 will spin more in an attempt to give better flight?

 

[Mike-Mike]

You are right. The difference is relative to what type of ammunition will shoot the best out of it.

A 1/9 will shoot most ammo well and is an "all purpose" twist.

A 1/7 is usually used in shooting longer / heavier bullets.

 

[daveyburt]

thanks!

1/9 sounds more general purpose, which is nice.

-appreciated!!

 

[qmmo]

Twist rates have a lot to do with bullet stabilization. Typically a 1 in 9 will reliably stabilize up to a 69-70 grain bullet (depending on barrel length). A 1 in 7 will stabilize a much heavier bullet. The old 1 in 12 that I have in one of my SP-1 Colt uppers is only good for 55 grain or less. The most common twist rate I've seen out there is 1 in 9, but recently there have been more and more 1 in 7 available (I think this is what is current in the military.....not sure though). This is about all I know on the subject (I load and sell 55, 62 and 69 grain). There are other very experienced high power rifle shooters on this forum that will have more info from a performance/accuracy perspective. Some are crankier than others but should definitely be listened to (bring a thick skin.....).

 

[Mike-Mike]

Qmmo is on the money. If you are not looking to get into competition I would go with the 1/9 it will shoot the cheaper ammo well. I have AR's on different twists. My carbines are 1/9 and my Match long range setup is 1/7.

 

[SecondAmdFirst]

Twist rates have a lot to do with bullet stabilization. Typically a 1 in 9 will reliably stabilize up to a 69-70 grain bullet (depending on barrel length). A 1 in 7 will stabilize a much heavier bullet. The old 1 in 12 that I have in one of my SP-1 Colt uppers is only good for 55 grain or less. The most common twist rate I've seen out there is 1 in 9, but recently there have been more and more 1 in 7 available (I think this is what is current in the military.....not sure though). This is about all I know on the subject (I load and sell 55, 62 and 69 grain). There are other very experienced high power rifle shooters on this forum that will have more info from a performance/accuracy perspective. Some are crankier than others but should definitely be listened to (bring a thick skin.....).

So it seems that the 1 in 7 twist is better. Why not use it on all barrels? I'm assuming in some situations the higher twist is actually negative?

 

[Mike-Mike]

So it seems that the 1 in 7 twist is better. Why not use it on all barrels? I'm assuming in some situations the higher twist is actually negative?

It's not about negative and positive its more about finding the right match with your ammo. I have shot 55'sr out of my 1/7 all day and they flew well, but never as good as 69's or even 77's would. For the regular AR guy either twist will do you well, If you really want to slow down and shoot for group size you may want to match the ammo to your barrel. If you want to shoot far 400-600yds you may want to carry a heavier payload and the 1/7 would do you a bit better. I am not a match shooter by any means.

 

[PatMcD]

Is it possible to get an AR barrel in the old 1-12 twist anymore? I would think for a varmint rifle, shooting light bullets, the slower twist would be preferred. I see 1-9 as neither here nor there.

 

[daveyburt]

thanks guys!

so, it seems like the higher the twist, the lighter the bullet needed for accuracy - good info!!
-or maybe better said, the lower the twist, the heavier...

after seeing a couple more replies, i'm thinking maybe a 1/7. though i'm a newb now, i very much enjoy precision at distance and maybe someday i'll compete. i'd just like whatever I buy to be good for the long haul so, that's a consideration.

again, appreciated!

PS: i try to wear a thick skin and thanks for the warning

 

[Mike-Mike]

Is it possible to get an AR barrel in the old 1-12 twist anymore?.

You know I had one with 1:14 AND I wish I still has it. I would be nice to load it up with some 30gr and see what it would do.

 

[gerrycaruso]

Twist rate goes by bullet length,not weight but since most bullets are lead or jacketed lead, going by weight usually works. Look up the greenhill formula and you can figure out the best twist for what you want to shoot. To confuse you even more, S&W has a new AR15 with a 1in8 twist. A couple of makers offer this rate because they believe the 1in8 gives accuracy as good as 1in7 with less fouling.

 

[Billsail]

Some studies claim that a faster twist rate will wear out the rifling quicker(more friction), and may shorten barrel life.

Bill

 

[WEJ 1163]

I just ordered some 75 grain Hornaday Match ammo. I've only shoot typical plinking stuff through a Colt Match Target that I just picked up. All 55 grain it didn't seem very accurate. Will this ammo be a better choice for the rifle it has a 1in 7 twist

 

[Mike-Mike]

I just ordered some 75 grain Hornaday Match ammo. I've only shoot typical plinking stuff through a Colt Match Target that I just picked up. All 55 grain it didn't seem very accurate. Will this ammo be a better choice for the rifle it has a 1in 7 twist

Yes, What types of groups where you getting with 55gr. I have a Colt target model and it does fairly well with 55 grains Like 1 MOA.

 

[WEJ 1163]

I would say with a solid rest and relatively good technique i was only getting 2" groups. I want to get better at off hand and compete in some local club events

 

[Mike-Mike]

I would say with a solid rest and relatively good technique i was only getting 2" groups. I want to get better at off hand and compete in some local club events

There are some high power guys here are they are very informative. yea, try some longer bullets. Also most over the counter longer and heavier bullet cartridges are higher quality in all regards to the surplus stuff.

 

[PatMcD]

To confuse you even more, S&W has a new AR15 with a 1in8 twist. A couple of makers offer this rate because they believe the 1in8 gives accuracy as good as 1in7 with less fouling.

I've had 1-8 barrels and they shot as well as my 1-7. For anything up to and including 80-grainers, the 1-8 is perfect.

 

[TomH]

So it seems that the 1 in 7 twist is better. Why not use it on all barrels? I'm assuming in some situations the higher twist is actually negative?

Supposedly 1/7 twist will make a 55gr tumble after a certain yardage. IIRC it is 500-600 yrds. A heavier round will perform better with 1/7. Both of my ARs are 1/7 and I have had no problems with them. But I also don't shoot in those ranges. For an all around rifle, 1/7 would be just fine.

 

[Rob Boudrie]

so, it seems like the higher the twist, the lighter the bullet needed for accuracy - good info!!

A higher twist is for heavier bullets. 1/9 is great for 55gr, but you need a 1/8 or 1/7 for the really heavy stuff.

A 1/8 is an excellent general purpose twist that will work well with 55gr bullets and also stabilize the heavy stuff.

 

[oneswithfunes]

Twist = 180 X D2/L

D= bullet diameter in "
L= bullet length in "

 

[boomerang]

Twist = 180 X D2/L

D= bullet diameter in "
L= bullet diameter in "

I may not be understanding properly.
If I substitute "L=bullet diameter in" for length, I still get strange numbers.
Can you elaborate?

 

[oneswithfunes]

Fixed it for you. My bad on the typo. The Greenhill formula is pretty old and outdated. Most people substitute 180 for the original value of 150 for speeds over 1800 fps. One problem is that it has no factor for velocity, hence it only gives you a value at the muzzle. It should only be used as a rough guide. Bryan Litz and Dan Lija both have very good reading on the subject. It get very heady.

 

[mike93lx]

The strange numbers are the twist rate as a fraction.

I.E. 1/8" is .125

 

[TonyDedo]

I would say with a solid rest and relatively good technique i was only getting 2" groups. I want to get better at off hand and compete in some local club events

At what distance?

 

[bpm990d]

I've shot 55gr M193 out of 1/7 barrels to 600 yards. It does not make the bullet tumble. It's plenty stable all the way out to 600.

Any tumbling issues at those ranges has to do with dynamic instability due to the bullet traveling through the transonic zone. Twist rate has nothing to do with the dynamic stability of a bullet.

B

 

[Coyote33]

You know I had one with 1:14 AND I wish I still has it. I would be nice to load it up with some 30gr and see what it would do.

Would make a nice coyote gun.

 

[oneswithfunes]

bpm99d,

I have been told that over stabilizing a bullet can lead to a increase in dispersion? Would this also lead to a increase in yaw relative to the target? What are your thoughts? this is something I am very interested in.

 

[bpm990d]

There are two types of stability when you talk about bullets, gyroscopic and dynamic. Gyroscopic stability is what most people talk and know about and is the result of the bullet being spun. The spin overcomes the overturning forces exerted on the projectile nose. Those forces are highest at the muzzle. As the projectile slows down, the overturning forces on the bullet nose decrease, but the spin degrades at a much slower rate than velocity. The result is that as long as a bullet is stable at the muzzle, gyroscopic stability will increase as the bullet goes down range.

Dynamic stability is more complex and has to do with how well a bullet negotiates the transonic area of its flight. The classic example of this is the .30 cal 168gr. bullet fired from an M-14. That combination just doesn't shoot accurately at 1000 yards because the 168gr bullet is dynamically unstable.

When you say stabilize, do you mean spin?

With match grade projectiles I don't think that you can spin them too much within reason. With ball bullets where the jackets are of poorer quality then the answer is yes. In the UK where they formerly used issue 7.62mm ball ammo for long range (800, 900 & 1000 yard) target shooting, they typically used 1/13 or 1/14 twist barrels. The reason was the jackets had problems with uniformity. One side of the jacket was a little thinner or thicker. When you spin these up out of a rifle, you get a wobbling because of the slight difference in mass. This is a picture for illustrating uniform case necks, but the same is true for bullet jackets

They found that spinning the bullets at the minimum required for stability resulted in greater accuracy. However match grade bullets by their nature have very uniform jackets. As an example of this, John Whidden, one of the best long range Palma shooters in the US uses a 1/10 twist barrel for shooting 155gr Palma bullets. Conventional wisdom was that he was spinning them too much, but it has not seemed to hinder his ability to win matches.

B

 

[oneswithfunes]

bpm990, Yes I mean statically or (gyroscopic) stable. When you say "dynamic stability" I am assuming you mean the tendency for yaw induced at the muzzle to reduce over time? I was wondering if using a twist rate much faster than needed for a stability factor of about 2 for any given bullet, could possibly increase yaw. I am wondering this because I have read that more twist than needed can increase dispersion. I am interested if this is true and if so why? Is it due increased yaw or others factors? I understand the internal factors affecting yaw.