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Ammo to avoid AR-15 1:9 twist

This is a discussion on Ammo to avoid AR-15 1:9 twist within the Ammo Dump forums, part of the Firearm Forum category; I am newbie to rifles bought first AR-15 last xmas I have been trying to get ammo when I can find it. I have been ...


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Old April 7th, 2013, 06:33 AM   #1
 
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Ammo to avoid AR-15 1:9 twist

I am newbie to rifles bought first AR-15 last xmas I have been trying to get ammo when I can find it. I have been reading trying to learn more about it and I am trying to figure out all the talk about 1:9 twist I understand what it means, my question is .. is there a weight/grain of ammo I should avoid if mine is 1:9 ? I mean I am not talking about doing competitioin shooting all I plan to use mine for is to target practice some, but main reason for getting it is to have in event of the Stuff happens scenarios play out.. any thoughts on ammo grain to avoid ?




Last edited by LAGun; April 7th, 2013 at 02:13 PM. Reason: Language Acronym
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Old April 7th, 2013, 07:09 AM   #2
 
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I'm not an expert by no means but you shouldn't have any problems shooting any normal ammo I would say anything besides like some 75gr. Match grade ammo which you'd probably only use in a competition. Any 55gr. Or 62gr.(most common) should be fine I went threw this same thing when I purchased my AR last summer ended up getting 1:7 twist cause I wanted a full mil spec rifle.
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Old April 7th, 2013, 08:03 AM   #3
 
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yeah I have bought lot of ammo recently most of it is 40 grain or 55 I do have one box of 75 grain.
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Old April 7th, 2013, 08:06 AM   #4
 
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You should be fine up to 75 grains. 40 grains may be a bit light.
ARFCOM has a FAQ:
What bullet weight can I shoot in my <1/9, 1/8> rifle?
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Old April 7th, 2013, 08:11 AM   #5
 
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most of what I have is 55 I have one box of 75 and 4 boxes of 40 what will too light do ?
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Old April 7th, 2013, 08:25 AM   #6
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What I have heard recommended for 1:9 twist is anywhere between 50gr to 62gr
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Old April 7th, 2013, 08:29 AM   #7
 
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<50= too light.
>62=Too heavy
Might shoot the 69g okay....
JMHO
Good luck!
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Old April 7th, 2013, 08:49 AM   #8
 
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I have an AR with 1:9 twist and regularly shoot 55 gr and 62gr with good results. As others have pointed out, the heavier bullets stabilize better with the faster twist rates. 75gr and 80gr. bullets are better suited it 1:7 or 1:8 twist rates
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Old April 7th, 2013, 08:49 AM   #9
 
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Quote:
Originally Posted by GREG50 View Post
most of what I have is 55 I have one box of 75 and 4 boxes of 40 what will too light do ?
An overly light projectile can 'over stabilize' by spinning the projectile faster than needed to stabilize it. That can, at long ranges, mess up accuracy. I am such a lousy shot anyway that I wouldn't notice it.
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Old April 7th, 2013, 08:56 AM   #10
 
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Re: Ammo to avoid AR-15 1:9 twist

1:9 twist is what most prefer because they shoot 55 gr, which is the most common on the shelves quite well. I shoot anything from 50 to 62 gr out of my 1:9 under 50 i shoot out of my 1:12 bolt gun.
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Old April 7th, 2013, 09:06 AM   #11
 
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Really good info everyone.
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Old April 7th, 2013, 09:20 AM   #12
 
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You need some correct information first of all.
Although we talk about different weights of bullets, WEIGHT doesn't mean poop in the world of stability.
Bullet stability is a function of length, RPM, and a bit of center of gravity/shape.
On this forum, between me and MZ5, we have had 4 DIFFERENT rifles that were 9-twist; all of them quite easily, and happily will stabilize a 77gr SMK to 700 yards.
A few good ballistics calculators can be found on the JBM site.

MV * 720 / TWIST = RPM

Everything is barrel smoothness & bullet construction dependent, but *most* bullets are happy around 200-220,000 rpm. "Over-stabilization" is another rather poor description of what is taking place. As it is used to describe nutation, and precession wobble; or the imbalance between centrifugal and centripidal force. Where core and jacket separation begins.

So remember the formula I gave you, and check out JBM, and read up on the green hill formula. Remember that nothing is absolute, but guidelines.
Either follow my link below, or call the bullet makers yourself and find the RPM limit for your particular bullet.

F.Y.I. Varmint Bullet RPM Limits - Shooters Forum
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Old April 7th, 2013, 09:40 AM   #13
 
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In these times any ammo that will go bang is better than none. If you are shooting at 20 yards the difference is not enough to worry about.
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Old April 7th, 2013, 09:52 AM   #14
 
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There are some misconceptions out there.

In the M16A2 and M855 (62 gr FMJ) ammunition, 1 in 9 twist was actually optimum based on the velocity and length of the bullet. 1 in 7 was adopted however as it was felt that it was important to adequately stabilize M856 Tracer ammunition which was substantially longer.

That was in a 20" rifle barrel generating about 3,100 fps, for a bullet spin rate of about about 240,000 rpm in a 1-9 twist barrel. With a 16" carbine you'll get about 2,800 fps at the muzzle with M855 and a spin rate of 224,000 rpm, about a 7% decrease in spin.

So as bullet weights get heavier (or more precisely as bullets get longer) barrel twist starts to make a difference sooner in a carbine than in a rifle as the velocity and spin rates are lower.

Those numbers compare with 2,750 fps and 283,000 rpm in a 1-7 twist 14.5" M4 barrel, where the 62 gr FMJ M855 round is still a bit over stabilized.

1 in 12 twist was used in the M16A1 and was optimum for the shorter 55 gr FMJ M193 round at 3,280 fps, giving a spin rate of 196,800 rpm. That increases to 262,000 rpm, but accuracy is still quite good in a 1-9 twist barrel. However in a 1-7 twist M16A2 20" rifle barrel at the same velocity, the M193 bullet is now being spun at over 337,000 rpm, a 71% increase over optimum rpm.

The problem with faster spin rates is that bullets are often less than perfect and have to transition from rotation around the center of form in the barrel to rotation around the center of mass outside the barrel. If the two are not identical, then there will be some yaw and gyroscopic precession as it transitions from one to the other, and the greater the rpm, the greater the precession that occurs, the greater the aerodynamic effects on the bullet and the greater the deviation from the original line of departure.

As a result, you'll see greater accuracy issues with lower quality bullets where there may be a greater frequency and/or magnitude of a difference between center of form and center of gravity.

The consistency of the base is also an issue as any inconsistency will result in gas exiting one side of the bullet before the rest, displacing the base of the bullet and again causing yaw and precession. Given the way FMJs are made with an open base, they tend to be much less consistent than a soft point or match type hollow point and are more prone to accuracy issues.

Consequently bullet style makes a difference with FMJs generally starting to have issues with over rotation before other bullet types.

-----

With M855 specifically, the yaw rates are pretty high (with the steel penetrator and FMJ design it's not real precise, especially when made by a low bidder shooting to just meet the minimum accuracy specifications) and they require about 100 meters to dampen to about 1/8the the original yaw amounts, by which point the bullet is in stable flight not in a straight-line but with a slight corkscrew path. Stretch a slinky all the way out and that's close to what the flight path would look like.

----

It all comes down to barrel twist, length and velocity.

In a 1-9 twist barrel the 62 grains FMJ is again optimum, not the upper edge of what works. The M855 projectile is nominally .907" long so rounds of similar length will stabilize equally well even if they are of similarly low quality.

An AR-15 with a 1-9 twist will shoot very well with a Sierra 65 gr SBT, Hornady 68 gr BTHP, Sierra 69 gr Match King or Nosler 69 gr BTHP. You'll want a velocity of at least 2,250 fps to ensure adequate stability.

It will also stabilize both the 75 gr BTHP and the 75 gr A-Max bullets adequately for accurate 600 yard shooting, but you'll want to keep the velocity over about 2,400 fps to stabilize these longer bullets. This is where a carbine may come up short.

A 1-9 twist barrel will aso stabilize bullets lighter and shorter than the 55 gr FMJ just fine as well. Most 50-53 grain bullets will perform quite well, and again the better quality match bullets will have fewer issues with other stabilization.

You can shoot lighter bullets in the 40-45 gr range, but you may find you want to slow them down to improve accuracy - which makes little sense, so just go with a heavier bullet.
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Old April 7th, 2013, 10:45 AM   #15
 
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thks guys for feedback I am thinking as lousy shot as i am that I will not notice it.
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