AR-15 Rifling; 1x7 vs 1x9

[7.62mm.ak47]

The M16 uses 1x7 and the AR-15's (generally) use 1x9. Does anyone know how much this actually effects shooting? From what I understand is that the 1x7 is better for long distance and higher grain rounds. Also why would AR-15 manufacturers use 1x9 instead of 1x7? Thanks guys.

 

[cougar1717]

1x9 is a compromise twist to use a greater range of bullet weights (really lengths). A wide range of bullets shoot well through the 1x9. Going to one extreme or another can limit what bullets shoot well in your rifle.

 

[DoubleTapDrew]

AFAIK the general rule is 1:9 is a good all around twist for 52-69gr while 1:7 can go up higher <80gr but may spin lighter bullets too fast causing them to wobble or come apart (esp. thin jacketed varmint bullets).

 

[benzy2]

Look at the market. The military isn't shooting varmints. They are shooting people. They have no use to optimize for a 36 gr varmint bullet that could be on the edge of blowing its jacket off with a 1:7 twist. Instead they want bullets that buck the wind and do more damage to human targets. The civilian world has a legitimate use for these light weight bullets and many people use their AR for varmint control. The faster twist (1:7) will stabilize the longer bullets that the 1:9 can't. The long range bullets typically need something faster than 1:9 and 1:7 seems to be the go to of late for the ultra long .223 bullets.

So for a civilian rifle, having a twist that shoots light weight bullets as well as some of the heavier bullets is a good compromise for the average shooter/hunter. If you are a long distance shooter or are hunting bigger game and want the heavier hunting bullets, the 1:7 twist fits that role well. On a general plinker, it really doesn't make much difference one way or the other. I would probably go with a 1:9 for a general purpose rifle and 1:7 for something I saw getting a more consistent dose of the heavier bullets.

 

[surfinUSA]

With 55 gr and under, even the old 1 in 12 inch twist is very accurate. And there are a zillion 55 gr rounds out there.

However, any new rifle I get will have a 1 in 9 just because the 62 gr rounds will become surplus just like the 55s did.

 

[7.62mm.ak47]

Ok thanks guys. So pretty much a 62 or 55 gr round will still hit out to 550yds also? I don't remember what we used in the Marines as far as round grain. We were pretty much just given our ammo and told to put 28 in our mags haha.

 

[TeamPrecisionIT]

I have a 1:7 twist and am now realizing I should have gone 1:9 due to the availability of ammo and bullets that are much easier to find in the 55-69gr range than >69gr rounds. The 1:7 is much better for >400 yard shooting with the 70+gr pills, but I don't get to shoot at the range enough to justify it.

So for a general purpose, gaming (other than long range, of course) and varmint hunting go with the 1:9. It could also be a plus if you plan on shooting mostly the surplus 55gr stuff.

For better long range stabilization with the heavier rounds, go with the 1:7 and plan to reload with more expensive bullets or buy much higher cost factory ammo.

Damian

 

[Farnham]

Just to be a chucklehead (as a Marine, you'll understand ), 1x8 is a possibility too. Mine shoots 77 grain and 62 grain well, not sure about the superlight bullets, ain't tried any.

 

[kwelz]

1/7 gives you a wider range despite what some people say. Unless you are shooting a specialty round of less than 50 grain you are better off with 1/7.

 

[HorseSoldier]

+1 1-7 has fine accuracy with everything the military loads. Unless you intend to shoot a lot of featherweight bullets or are fine tuning a rifle for hair splitting from a bench rest, it doesn't really make sense to go with anything else.

 

[MIL-DOT]

I agree. I was also concerned with twist rates, but nearly all of what I dug up said that my 1/7 rifle was fine with 55 gr. bullets, and my subsequent shooting experience supports this,as well.
I am very unlikely to ever shoot any ultra-light ammo. All of what I have is 55 and 62 grain, and that's the majority of what I see around.
I think 1/7 is the way to go.

 

[briansmithwins]

I've shot 45gr loads thru 1/7 barrels wi th no problems. I was seeing 2.5MOA with 55gr XM193 and 1.2MOA groups with 75gr PPU OTM ammo.

Generally, 1/7 gives you the most flexability. BSW

 

[jonboynumba1]

basically if you want to shoot Hornady TAP 75gr and similarly weighted match grade ammo a 1:7" twist is what you want. If you want to keep it 62-40gr range 1:9" is the best compromise....I've heard it said years ago that the tighter 1:7" would have negative effects on some lighter SP ammo. These days I doubt that is the case...but with tighter rifling comes increased friction and heat which means wear...I preffer a 1:9" chrome lined barrel (including the chamber...which should be true 5.56 NATO spec. In a carbine I feel it's best...in a 20" match gun I might well preffer a non lined 1:7" perhaps with RRA's .223 Wylde chamber. But I'm more into carbines. Only 1:7" 20" rifle I shoot is the bosses Colt NM...and it's not my favorite (but groups well!) It's all what you want...but if you shoot whatevers cheap and decent and don't need 75gr bullets for long range target or killing BG's get a 1:9...it's become the most common.

It's also been said the older looser 1:12" and 1:16" twists where the most lethal with 55gr ball ammo (allowed ammo to yaw/tumble upon pentration better...much like 5.45x39 does)....but the newer heavier 62gr meant a tighter rifling was needed...I'd have gone with the 1:9" but uncle sam didn't ask me...I consider 75gr past the balance point for that cartridge IMHO. Also FWIW -Hornady makes TAP in lighter versions that work fine in 1:9" if that's what you want for bump in the night use.

 

[rcmodel]

The military 1/7 was optomized for the very long tracer rounds in use by our NATO friends in SAWs at the time.

Were it not for that, I think we would have a more lethal rifle if they had used 1/9 on the M16 and especially on the shorty M4 with the 62 grain load they use now.

The early M16 with 1/12 barrel was noted for it's destructive killing power with 55 grain bullets in Vietnam.
Thats because they would tumble and blow up like a varmint load.

Now with 1/7, the dang 62 grain bullets are too stable, and don't want to tumble at much past 200 yards velocity, and just drills .22 holes on through.

rc

 

[HorseSoldier]

The military 1/7 was optomized for the very long tracer rounds in use by our NATO friends in SAWs at the time.

I'm not sure if anyone in NATO besides the US was using 5.56mm at all as a standard issue at the time. The US was party to the plan to upgrade 5.56mm performance, including the adoption of the M856 tracer.

It may not have been your intent, but a lot of commentary about 5.56mm carries the implication that SS109 or even 5.56mm itself were somehow inflicted upon the US by our European allies.

 

[rcmodel]

Well, as I understand the history of it.
The XM249 SAW was developed by Fabrique Nationale of Belgium and first unveiled in 1974.
Known in Europe as the FN Minimi, its development took place well before the NATO 5.56mm Second Caliber Standardization Agreement (STANAG 4172) to assure commonality of ammunition.

The M855 5.56mm ball cartridge uses the Belgian SS109 bullet which weighs 62 grains and contains a hardened-steel penetrator frontal core and lead base encased in a copper jacket.

The M249 SAW was adopted into U.S. military service in 1982, well after the FN Minimi SAW & M855 ammo was in use by more then one other NATO country.

rc

 

[mshootnit]

1 in 9 is better

I like the 1 in 9 twist better. It groups better with ammo ranging from 45 to 62 grains which is nearly all the sporting shooter needs. The lighter loads have a velocity advantage in the carbine.
1 in 7 is a modification intended for those shooting primarily 69 grain ammo or higher.

I once shot a coyote with a 1 turn in 12 inches twist rifle at 235 yards with a 60 grain projectile, so don't believe the crowd that 1 in 9 is too slow for anything over 55 grain.

 

[M&PVolk]

Give me 1:9 twists for the better selection of commonly available ammo out there. The cheapest and most abundant ammo I have seen is in the 50, 55 and 62 grain variety. If you aren't shooting past 100 yards with it, my understanding is the 1:7 is just fine with these rounds, but if you reach out to 200+ the groups really open up.

 

[cottonmouth]

I like a 1 in 9 just fine, I have a 20 inch Hbar that I wish had a 1 in 12 so I could go with the lite bullets.

J.B.

 

[ccjcc81]

Educate me, what happens if I were to shoot a <80grn bullet through my 1/9?

 

[Matt304]

Educate me, what happens if I were to shoot a <80grn bullet through my 1/9?

Uh, you said under 80 grain. So that could mean a 50 grain. Maybe you meant to just say "what happens if I were to shoot an 80grn bullet through my 1/9?"

If you shot an 80 grain bullet through a 1/9, it just wouldn't stabilize very well. It may slightly tumble before hitting a 100 yard target, or it may just be very inaccurate due to slight wobble.

The problem is that a long pointed tube shape is not stable naturally. It is why rockets need to have fins at the rear. If you try to throw a football without spinning it in a spiral, it will wobble and tumble. This is because naturally, the center of pressure is in front of the center of gravity, or too close to neutral. The denser the material, the more centrifugal force it generates when it spins. So two bullets of the same size, one of aluminum and one of lead, the one made of lead generates more centrifugal force when spun at the same speed as the aluminum bullet. That means lighter bullet materials require them to be spun faster. This force tries to pull outwards in all directions, and sort of locks the bullet in its place while it travels through the air. We used to shoot a frozen pond with a 9mm FMJ. First you would shoot a hole in the thick ice, then you would shoot sideways straight into the ice. About 1 out of 3 times, the bullet would pop up out of the shattered ice, and spin like a top on the ice. They had to be spinning at about 50,000RPM--they would sit there for about a minute on the ice spinning, and you could not knock them down!

There is a prediction that's called the Greenhill formula or equation. Here is a link to a calculator so you can play around with it to predict minimum stable twist. http://kwk.us/twist.html

 

[C-grunt]

Like was stated earlier the 1in7 was actually adopted for the longer tracer round. A 1in9 will stabilize a 62grn round just fine. If you are going to shoot cheap 55 and 62 grn rounds for the most part, a 1in9 will serve you well.

A 1in7 will shoot the heavier 70+ grain bullets better, but that doesnt mean a 1in9 wont stabalize the heavier rounds up to 77rgn or so, its just more likely that it wont.

 

[dogrunner]

RE: the 1/8 Farnam mentioned. Have a stainless Wilson on a RR in 16" that will hold tight (less than a moa) with 40 gr. Sierra hp's.

 

[Kwanger]

IMO, unless you are either a varmint hunter who uses very light bullets, or a long range target shooter who uses the heavy ones, either 1 in 9 or 1 in 7 will be just fine for 99 percent of ammo that the average person will shoot (which is 55gr or 62gr). Either twist rate will shoot these weights just fine.

 

[ccjcc81]

Uh, you said under 80 grain

LOL, I racked my brain for 2 minutes trying to remember learning in elementary school "which way the aligator attacks."

If you shot an 80 grain bullet through a 1/9, it just wouldn't stabilize very well. It may slightly tumble before hitting a 100 yard target, or it may just be very inaccurate due to slight wobble.

The problem is that a long pointed tube shape is not stable naturally. It is why rockets need to have fins at the rear. If you try to throw a football without spinning it in a spiral, it will wobble and tumble. This is because naturally, the center of pressure is in front of the center of gravity, or too close to neutral. The denser the material, the more centrifugal force it generates when it spins. So two bullets of the same size, one of aluminum and one of lead, the one made of lead generates more centrifugal force when spun at the same speed as the aluminum bullet. That means lighter bullet materials require them to be spun faster. This force tries to pull outwards in all directions, and sort of locks the bullet in its place while it travels through the air. We used to shoot a frozen pond with a 9mm FMJ. First you would shoot a hole in the thick ice, then you would shoot sideways straight into the ice. About 1 out of 3 times, the bullet would pop up out of the shattered ice, and spin like a top on the ice. They had to be spinning at about 50,000RPM--they would sit there for about a minute on the ice spinning, and you could not knock them down!

There is a prediction that's called the Greenhill formula or equation. Here is a link to a calculator so you can play around with it to predict minimum stable twist. http://kwk.us/twist.html

Wow, thats a great answer. You explain things very well. Thanks!