AR-15 Rifling; 1x7 vs 1x9


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7.62mm.ak47
July 14, 2010, 06:23 PM
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.

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cougar1717
July 14, 2010, 06:31 PM
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
July 14, 2010, 06:32 PM
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
July 14, 2010, 06:53 PM
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
July 14, 2010, 09:21 PM
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
July 14, 2010, 10:12 PM
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
July 14, 2010, 10:32 PM
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
July 14, 2010, 11:17 PM
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
July 15, 2010, 02:18 AM
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
July 15, 2010, 04:35 AM
+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
July 15, 2010, 10:17 AM
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
July 15, 2010, 10:20 AM
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
July 15, 2010, 04:12 PM
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
July 15, 2010, 04:22 PM
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
July 15, 2010, 04:39 PM
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
July 15, 2010, 05:59 PM
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
July 15, 2010, 08:14 PM
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
July 15, 2010, 10:20 PM
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
July 15, 2010, 11:34 PM
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
July 16, 2010, 12:58 AM
Educate me, what happens if I were to shoot a <80grn bullet through my 1/9?

Matt304
July 16, 2010, 03:22 AM
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
July 16, 2010, 03:38 AM
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
July 16, 2010, 10:22 AM
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
July 16, 2010, 11:00 AM
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
July 16, 2010, 05:33 PM
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!

MistWolf
July 16, 2010, 06:27 PM
Actually, there's a bit of inaccuracy in that answer. The weight of the bullet does not determine twist rate. It's bullet length. A long aluminum bullet doesn't need a faster twist than a short lead bullet because it's less dense, it needs a faster twist because it's longer. Even a long heavy bullet needs more twist.

A tighter twist doesn't keep a bullet from destabilizing on impact with a body. Remember, you don't want a bullet to tumble. Tumbling is when the bullet tumbles end over end in flight. It's what gives you keyholing in a target- if the bullet even hits the target.

What you want is the bullet to destabilize when it hits a body. This occurs because the bullet has made a sudden transition from traveling through air to traveling through water. It causes the bullet to yaw severely. High impact velocities are needed for this. The difference of RPM of the bullet from a 1:12 or a 1:7 isn't enough to prevent a bullet from destabilizing.

The 55 gr bullet used in Viet Nam wasn't devastating because the rifles had a 1:12 twist, it's because the war was fought at contact ranges where impact velocities were still very high. Some bullets bent and even broke creating secondary projectile wound tract.

The new US rounds were designed specifically to destabilize and break apart at high impact velocities from a 20" rifle. Now our military is using the M4 carbine with a 14.5" barrel with a significant reduction in muzzle velocity.

Strangely enough, the ammunition works fine in rifles even though they have 1:7 inch twists.

You have to decide what you want to use your AR for. If it's a carbine, you might as well get the 1:7 twist. With a shorter barrel, you get less muzzle velocities and the bullet will have a lower RPM. More than likely you'll be plinking with it and shooting cheap 55 gr FMJ. It will work fine. But if you decide to use something heavier (which is generally longer) the 1:7 will be more likely able to stabilize it. If you ever decide to sell your AR, with a 1:7 twist will hold it's value better.

A rifle with a 20" barrel should work well with a 1:9. The 20" has higher muzzle velocities and the bullets will have a higher RPM. After all, it isn't the twist that stabilizes the bullet, it's how fast it's spinning

Matt304
July 16, 2010, 07:10 PM
Always somebody who will try to top the next person around here, no matter what.

I will follow suit so I don't break this vital trend. ;)

There's giant inaccuracy in that answer just given. Notice I did not say bullet weight, I said density!

Material density has an absolute effect on stability. Stability is a function of center of gravity in relation to center of pressure. A bullets density plays in part with its shape to determine where the center of gravity will be. The length only measurement sometimes used in the Greenhill formula is due to the fact that it uses a very simplified formula to predict bullet shape and hence center of pressure! Do you think an advanced fluid modeling program would leave out material density when modeling a bullets stability?

The site I linked above is down at the moment, but view the cached version of it on google. Even it knows to take into account the material density in the prediction of stability. It does matter, but a lot of bullets are believed to be of a very similar density, so to make it easy on the less mathematically inclined, they use a default value and only ask for length.

Just because your understanding of something is in lamens terms doesn't mean its existence in reality represents that same model of simplicity! :p

Remember that when you provide a caliber, weight, and then you provide a length, you can use a generic bullet shape to derive density itself. If it doesn't ask for weight, than it is using a predefined density value.

MistWolf
July 16, 2010, 08:09 PM
Sometimes it's easy to forget all the principles still apply when you pare things down to practical application. I stand corrected

Matt304
July 16, 2010, 08:50 PM
No harm done.

I like helping people understand the science of things and make sure they understand why things work, and not just "that they work". Knowledge is power. :)

tju1973
July 16, 2010, 10:55 PM
I have an 18" AR w/1/7" twist...it shoots 55grn-77grn without any issues...never shot lighter weight stuff out of it, but someone on ARF told me he shoots 45grn (and lower?) out of his 1/7 and has no issues at normal ranges...
ymmv

M&PVolk
July 17, 2010, 11:11 AM
Again, distance plays a role here. If you are shooting short range only (less than 100 yards) a 1:7 will be versatile for you. If you shoot any longer than that, the 1:7 will need longer bullets to achieve good accuracy. The 1:7 will cause 50-55 grain bullets to group very large past that distance.

1:9 twist gives you excellent accuracy out to 350 with the most common grain ammo available, that being 50-68 grain, but will likely keyhole bullets past that bullet length. Remember, length is the key, but in practical execution, you will find longer bullets are typically a heavier grain.

I know 1:7 is the current trend for "mil-spec" but I would prefer a 1:9 or ideally, a 1:8 twist anyday. It is cheaper to feed a 1:9 and get good accuracy, and you can still reach out to a good distance and make match grade custom loads. The 1:7 is marginal with the most readily available ammo, and the longer bullets get pricey.

RIATAC45
July 18, 2010, 08:14 PM
I am new to the AR game so I don't have a real clear understanding of the twist vs bullet weight subject. Would you fine gentlemen point me int the right direction, I would like to know what weight range I should use in my 24" 1 in 9" colt hbar. Any help would be greatly appreciated.

dom1104
July 18, 2010, 08:20 PM
another thing to think about, if you are going to put a 22lr kit into the gun, 1/9 is far better than 1/7.

my 1/9 barrel is working very well for me, been 1st in 22lr steel matches quite a few months in a row here.

I could not be more happy with it, 100% reliable with close to 10k rounds now.

something to think about.

benzy2
July 18, 2010, 08:26 PM
I am new to the AR game so I don't have a real clear understanding of the twist vs bullet weight subject. Would you fine gentlemen point me int the right direction, I would like to know what weight range I should use in my 24" 1 in 9" colt hbar. Any help would be greatly appreciated.What do you want to shoot? You can shoot any weight you like, but know that bullets heavier than 69 grains may or may not stabilize. You may have bad accuracy and keyholing with heavier bullets. Every barrel is different though. Some will stabilize bullets slightly heavier than 69gr and others won't. You have to try a few out to see what works and what doesn't. On the low side, you can shoot basically anything you can find. Some of the 36gr varmint bullets may need loaded down a little as 1:9 could be fast enough to blow a jacket off, but I'm not sure on that and if you slowed them down a little they should stay together just fine. Out side of the absolute lightest weight/thinnest jacket stuff, there isn't much to worry about other than what is most accurate. If you are shooting at varmints, something in the low 50's grain wise meant to expand rapidly (like the V-max) would be a good option. For paper punching, the Sierra Match Kings in various weights all seem to shoot well. The flat base lighter bullets seem to shoot best for me at closer ranges, but the boat tails typically have better ballistics and shoot better at longer distances and in the wind.

I'll be honest, I've never really experienced a problem where a barrel with a faster twist shot lighter weight bullets poorly(if they stay together). I haven't seen the overstabilized problem. The problems I have seen are when you drive them too fast, they spin too fast and the jackets come off. I have heard many times to buy just enough twist to stabilize the bullet you want, but shooting faster twist barrels with quality lighter weight ammo hasn't shown to be very detrimental to accuracy, more to losing jackets when you go too far.

Bartholomew Roberts
July 18, 2010, 08:32 PM
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.

Twist rate has nothing to do with the bullet's stability in a mostly liquid medium like flesh. The 55gr from the 1:12 barrel happens to not only have pretty much the same velocity floor for reliable fragmentation as 62gr from a 1:7 barrel (2,700fps), it also has about the same percentage of where it yaws (or whether it yaws at all) - about 15% yaw before 4", 15% yaw after 6-7 inches, and about 70% yaw between 4-7 inches for both rounds.

You need a super high twist rate (like a machine screw) to put enough spin on a bullet that it would stay stable in the transition from air to a much, much denser medium. I don't know how this myth got started; but it has no basis in science.

RainDodger
July 18, 2010, 08:38 PM
Not to beat a dead horse, but my AR is 1:9. I reload primarily 60 grain Sierra "Varminter" (#1375) bullets. Works great, and it's plenty accurate.

Just for comparison's sake... my Browning B-78 .22-250 (same diameter bullet as the 5.56) is a 1:14 twist and it seems to like 52-53 grain bullets. And I don't go heavier than that. I would say that you'll be fine with either barrel. My choice would depend on my use for the rifle. If it was a SHTF rifle, 1:7, because I'd be shooting 60+ grain bullets and nothing lighter. If it was a GP rifle, 1:9, so I could take advantage of both bullets.

RIATAC45
July 18, 2010, 08:45 PM
I am mainly looking for accuracy, our range is 600 yds. I have yet to try anything past 100yds with it, I would like to shoot 300 if possible.

TOU
July 19, 2010, 03:46 AM
Okay...I have a Daewoo DR-200 that I want to SBR. It came with a 1:12 twist, the question is for me...how short do I go. My intended use for this rifle will likely never reach 200 yards & is more likely to be 100-150 yards and less. I want to be able to shoot standard surplus ammo. So what length of barrel should I go down to & stick with what ammo/bullet?

Thx

TOU

benEzra
July 19, 2010, 12:18 PM
The M16 uses 1x7 and the AR-15's (generally) use 1x9.
Actually, it looks like a pretty even split in the AR market between 1:7 and 1:9, with the pendulum swinging toward 1:7.

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.
1:7 works great with 55-69gr. You only need 1:9 if you want to shoot the super-lightweight 40gr varmint bullets.

So pretty much a 62 or 55 gr round will still hit out to 550yds also?
Yes. The lighter bullets will carry a little less energy at 600 yards than the heavier bullets due (due to a lower ballistic coefficient), but they'll get there.

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