AR-15 Rifling; 1x7 vs 1x9

[MistWolf]

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]

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!

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]

Sometimes it's easy to forget all the principles still apply when you pare things down to practical application. I stand corrected

 

[Matt304]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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.