AR15.Com Archives
 1 in 7 vs 1 in 9 twist rate.
red_cedar  [Member]
10/22/2009 8:37:18 AM
I,m in the market for my first ar carbine. Have been doing the reasearch and most of the barrels from the various manufacturers have a 1 in 9 twist rate. The colt 6920 has a 1 in 7. What is the real difference to the various twist rates.

Thanks guys.
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mathecb  [Team Member]
10/22/2009 8:44:44 AM
1 in 7 is "milspec". It is used to shoot heavier bulllets from 62 to 77 grains.(approx) The 1 in 9 is used to shoot medium weight bullets. A 1 in 9 should shoot 45-65 grain with no problem. Most of what you buy in stores these days is 55 grain and maybe some 62 grain. The 1 in 9 twist is perfect for these but the 1 in 7 usually has good results too. Most top-tier manufacturers use 1 in 7 twist, thats what I would go for.
Frens  [Member]
10/22/2009 8:45:34 AM
the 1/7 can handle heavier bullets up to 77gr while the 1/9 up to 69gr.

if all you use is surplus ammo (55 or 62gr) the 1/9 will serve you well.
billyjoebob  [Member]
10/22/2009 9:27:03 AM
It's worthy to mention that a 1 in 9 will shoot heavier bullets...they just probably won't stabilize as well and you could lose some accuracy at long ranges. But the weapon will function just fine.

walt_l  [Team Member]
10/22/2009 9:41:18 AM
BTW Colt 6920 comes in 1/9 and 1/7 both. I have both. Not sure if other twist rates are availble, but those tow for sure
Eric802  [Team Member]
10/22/2009 9:56:28 AM
If you think you might want to shoot 75gr+ ammo, go with 1/7. Many 1/9's will shoot 75gr Hornady BTHP ammo, some will not. 75 grains seems to be the go/no go line for how heavy a bullet a 1/9 barrel will stabilize.
peecmkr45  [Member]
10/22/2009 10:22:40 AM
If you plan on wanting to get accuracy from military 5.56 tracer then you would want 1 in 7 twist.(That's the original reason the military uses it.) Otherwise the 1 in 9 is GTG.
red_cedar  [Member]
10/22/2009 12:37:47 PM
So the 1 in 7 twist should handle the full range of bullet weight 55 to 75 just fine then?

Im actualy looking a J& T DS-4 kit where one can get either one of the mentioned twist rate barrels.
Eric802  [Team Member]
10/22/2009 12:45:15 PM
1/7 will handle up to 80gr. If you're looking for the greatest versatility, 1/7 is the way to go.
Frens  [Member]
10/22/2009 12:51:52 PM
+1 up to 80gr

in my previous post I said up to 77 because I dont think a 80gr bullet will fit in the magazine.
COMMAND450  [Member]
10/22/2009 1:03:45 PM
are there any trade offs.

why did colt use 1/12 twist on 20" barrels then?

barrel life? velocity and muzzle energy?


they knew what they were doing back then about twist rates, so why did they not just go with 1/7
9divdoc  [Team Member]
10/22/2009 1:07:37 PM
Originally Posted By COMMAND450:
are there any trade offs.

why did colt use 1/12 twist on 20" barrels then?

barrel life? velocity and muzzle energy?


they knew what they were doing back then about twist rates, so why did they not just go with 1/7


They went to 1 in 12 because the 1 in 14 didn't work out so good
Frens  [Member]
10/22/2009 1:11:17 PM
from the Retro forum
While the standard 1/14 .224 barrel twist was quite successful in commercial firearms it did not serve the AR-15 well. M193 ammunition utilizes a boat tail projectile rather then the more common flat base. Boat tail bullets have less bearing surface to engage the rifling of the bore then flat base bullets of the same weight. The result was that the 55 grain FMJBT bullets were on the edge of stability when fired from the AR-15. Accuracy testing conducted by the US Army, US Air Force, and the NRA showed unacceptable accuracy with the 1/14 twist. Because of these factors a twist change to 1/12 was approved on July 26th, 1963.


M16A2/A4 barrels are marked C MP 5.56 NATO 1/7. The C MP marking means the same as before. The NATO 1/7 specifies the rifle is designed for NATO standard ammo rather then M193 as used with earlier rifles.


1/12 was good for standard US ammo (M193 55gr)
1/7 is good for standard NATO ammo (M855 62gr)

1/12 twist doesnt stabilize bullets heavier than 55gr....62gr will keyhole on the target.
COMMAND450  [Member]
10/22/2009 1:28:44 PM
Originally Posted By Frens:
from the Retro forum
While the standard 1/14 .224 barrel twist was quite successful in commercial firearms it did not serve the AR-15 well. M193 ammunition utilizes a boat tail projectile rather then the more common flat base. Boat tail bullets have less bearing surface to engage the rifling of the bore then flat base bullets of the same weight. The result was that the 55 grain FMJBT bullets were on the edge of stability when fired from the AR-15. Accuracy testing conducted by the US Army, US Air Force, and the NRA showed unacceptable accuracy with the 1/14 twist. Because of these factors a twist change to 1/12 was approved on July 26th, 1963.


M16A2/A4 barrels are marked C MP 5.56 NATO 1/7. The C MP marking means the same as before. The NATO 1/7 specifies the rifle is designed for NATO standard ammo rather then M193 as used with earlier rifles.


1/12 was good for standard US ammo (M193 55gr)
1/7 is good for standard NATO ammo (M855 62gr)

1/12 twist doesnt stabilize bullets heavier than 55gr....62gr will keyhole on the target.







thanks for digging that up.

so why would they not just go 1/9 back then?

do the 1/7 barrels fair well with 55gr ammo?


would not the 1/7-1/9 barrels have shorter life spans?

Frens  [Member]
10/22/2009 1:34:37 PM
Originally Posted By COMMAND450:

thanks for digging that up.

so why would they not just go 1/9 back then? because the tracer ammo is also in use (75 or 77gr IIRC) and the 1/9 is not good enough with such heavy bullets

do the 1/7 barrels fair well with 55gr ammo? I use 55gr regularly with my 1/7 and I've no complains


would not the 1/7-1/9 barrels have shorter life spans? I'd think so



red_cedar  [Member]
10/22/2009 2:04:11 PM
Originally Posted By Eric802:
1/7 will handle up to 80gr. If you're looking for the greatest versatility, 1/7 is the way to go.



Thats what im looking for. Looks like 1 in 7 is the way I will go.

Thanks everyone for your time and knowledge.
GJim  [Member]
10/22/2009 3:18:42 PM
Although we talk about different twists for different weights, I believe the real issue is bullet length. That's probably why the military tracer needs a 1/7 even though it's not particularly heavy.

We are already beginning to see restrictions on the use of lead bullets in some areas, and that will probably get worse before it gets better. If you end up having to shoot non-lead core bullets out of your AR some day, the 1/7 twist might work better for those "lightweight but long" bullets.
kcr121  [Team Member]
10/22/2009 5:41:27 PM

Originally Posted By walt_l:
BTW Colt 6920 comes in 1/9 and 1/7 both. I have both. Not sure if other twist rates are availble, but those tow for sure

Not true. If you have a Colt with a 1/9 twist barrel, it's a model other than the LE6920.

Gregory_K  [Team Member]
10/22/2009 6:12:30 PM
1/12 if i want heavy i take a 7.62 to the range.
Gregory_K  [Team Member]
10/22/2009 6:12:55 PM
Originally Posted By kcr121:

Originally Posted By walt_l:
BTW Colt 6920 comes in 1/9 and 1/7 both. I have both. Not sure if other twist rates are availble, but those tow for sure

Not true. If you have a Colt with a 1/9 twist barrel, it's a model other than the LE6920.



that would be the 6721.
Frens  [Member]
10/22/2009 6:13:29 PM
there was a 6920 with 1/9... I saw a pic...I'm quite sure the model number was different but it did exist

ETA:
found the pic I was talking about and the model number was 6922

SuperiorBarrels  [Industry Partner]
10/22/2009 7:11:01 PM
1:8 will stabilize the Sierra 80 grain bullets, but needs right around 250-300 yards to go to sleep.

I have several 1:7 barrels with Hard Blue stabilize bullets up to 115 grains, but needs 300 yards to go to sleep.
Nichonator  [Member]
10/22/2009 8:01:15 PM
My 1-9 Armalite SPR will shoot 69gr Black hills red box pretty accurate. It really likes the 62gr federal Fusion better.

My back yard is measured off to just over 800 yards. I shoot my Armalite at 400 and have yet to have a bullet key hole.

This same bullet is very erattic accuracy wise out of my 20" Colt HBAR 1-12" but the bullets dont keyhole even at 400 yards the accuracy just sucks.

If this concept of keyholing is correct how come the 210gr .308 bullets I handload dont keyhole out of my M-1 a designed for 165gr bullets either??

Nichonator  [Member]
10/22/2009 8:21:46 PM
Also to amend the comment that the HBAR accuracy sucks at the 400 yard mark i can still hit the target. Even if the 69 gr were all i had i would just shoot more bullets to compensate for my old vision and the heavy bullet weight. And with a 30rnd mag in place it kind of negates the accuracey part ,as my son has proved 30 rnds latter there are still a shit load of holes in the target. Kind of the minute of man theory.
lazyengineer  [Team Member]
10/22/2009 8:50:35 PM
I've always been disappointed that the ammo oracle doesn't address this. The question comes up again and again. 1/7 twist is mil-spec for the sole reason that this is necessary to stabilize the tracer round, and do so on the coldest day imaginable at the coldest place on the Earth. And the tracer round is really long.

As to twist rate, IMHO unless you plan on shooting exotic long bullets (which you probably won't), then the 1/9 is better tuned for the 55 gr bullets you are more likely to shoot. 1/7 will work too, and most folks don't seem to notice the difference on paper. At one time Bushmaster was marketing that 1/9 is superior because you have a longer barrel life. Don't know if that's really true, but it always gave me pause.

If you shoot exotic light weight bullets (<50 gr), then you have elevated risk of disintegrating them. So saying 1/7 is the most versital isn't entiiirely true. A tumbling 80 gr bullet is better than a 40 gr bullet that turned to dust IMHO. And even for the heavies, match barrels rarely go as fast as 1/7. RRA uses 1/8.

I sometimes wonder if there is too much emphasis on the heavy bullets these days. Part of the reason .223 was selected is because it's a wicked fast round. Shooting heavy bullets in this diameter negates that. Heavy is appropriate for match shooting, where the weight helps the bullet buck the wind for those 600 yard shots. But for a general use AR shooting ball ammo - the ammo oracle made it clear to me that 55 gr is more lethal over a longer range than heavy. To take it a step further, 40 gr bullets are really wicked fast, but you can't shoot something like that in a 1/7.

My recommendation - if you have to ask, you're probably not a high-power match shooter, so go 1/9.



Nichonator  [Member]
10/22/2009 8:55:45 PM
Originally Posted By lazyengineer:
I've always been disappointed that the ammo oracle doesn't address this. The question comes up again and again. 1/7 twist is mil-spec for the sole reason that this is necessary to stabilize the tracer round, and do so on the coldest day imaginable at the coldest place on the Earth. And the tracer round is really long.

As to twist rate, IMHO unless you plan on shooting exotic long bullets (which you probably won't), then the 1/9 is better tuned for the 55 gr bullets you are more likely to shoot. 1/7 will work too, and most folks don't seem to notice the difference on paper. At one time Bushmaster was marketing that 1/9 is superior because you have a longer barrel life. Don't know if that's really true, but it always gave me pause.

If you shoot exotic light weight bullets (<50 gr), then you have elevated risk of disintegrating them. So saying 1/7 is the most versital isn't entiiirely true. A tumbling 80 gr bullet is better than a 40 gr bullet that turned to dust IMHO. And even for the heavies, match barrels rarely go as fast as 1/7. RRA uses 1/8.

I sometimes wonder if there is too much emphasis on the heavy bullets these days. Part of the reason .223 was selected is because it's a wicked fast round. Shooting heavy bullets in this diameter negates that. Heavy is appropriate for match shooting, where the weight helps the bullet buck the wind for those 600 yard shots. But for a general use AR shooting ball ammo - the ammo oracle made it clear to me that 55 gr is more lethal over a longer range than heavy. To take it a step further, 40 gr bullets are really wicked fast, but you can't shoot something like that in a 1/7.

My recommendation - if you have to ask, you're probably not a high-power match shooter, so go 1/9.





Amen brother!! I shoot a shit load of PMC and winchester 55gr in everything i have from 1-7 to 1-12 and it works. Is it better or worse than one or the other?? Maybe if your counting inches and not bullet holes.

forever4  [Team Member]
10/22/2009 9:04:48 PM
I reload all my ammo for the 5.56. For the 50-62 gr bullets the 1-9 works great. I have skipped the 1-7 in my carbines. For my varmint rifle I use a 1-8 for my 77gr Match Kings. On the box they come in Sierra tells you they are only for 1-7 or 1-8 twists. They also don't recommend them for hunting. (Heavy walls) For my bolt gun, a Model 700 (.223) a 1-12 is used for the 40-55 grain varmint and target rounds. That rifle will not do well at all with heavier bullets.

Each twist has its place. The 1-7 will take a varmint bullet and explode it do to the 300,000 RPM spin on the thin jacket of that type bullet. Military rounds are heavier walled. There is NO one perfect spin rate, even for a given caliber. I have even seen target rifles with stuff like 1-7 1/4 or 1-9 1/2 type rates. Competition shooters go to extremes to make rifles that are maxed out. For your day to day garden verity AR either 1-7 or 1-9 will work. If you are using run of the mill 55 - 62 gr. bullets then either will work fine. 1-9 would be most common in your day-to-day commercial rifles.

50 gr to 55 gr bullets are common in this caliber as they will shoot pretty well with the common twists from 1-7 to 1-12

There is a ton of info on this out on the web. You will also find all sorts of personal opinions on this subject. Seems as much a black art as it is a science.
Nichonator  [Member]
10/22/2009 9:22:53 PM
I think it is a little over science. The 55gr ballistic tip bullets shoot well in everything from my 1-7 carbine to my 1-12 Weatherby Vanguard and Colt HBAR, Its a varmint bullet. The 55gr. will shoot well enough in everything. So if you must stock up this weight makes the most sense for those of us with multiple weapons, the FMJ is a better all around choice of course than the ballistic tip. I shoot long range more than most and it is my prefered round. Granted the 62 and 69 are better performers for me at the 300-400 yard mark. But again this is minute of man talk not MOA.. Molon will quote the penetration stats and his info is pricless and very well researched and documented. That being said all the way around at under 100 yards (90%+ of the documented police shootings with this round) the 5.56 / 223 55gr round is a proven killer time and time again. If you do your part and put it in the head or upper Torso there done. The 55gr .223/ 5.56 round is very deadly especially up close. just ask any Trauma surgeon or your local County medical examiner.
gee223  [Member]
10/22/2009 9:56:47 PM
Originally Posted By red_cedar:
So the 1 in 7 twist should handle the full range of bullet weight 55 to 75 just fine then?

Im actualy looking a J& T DS-4 kit where one can get either one of the mentioned twist rate barrels.


Mine is their DS-4 upper with the 4150, 1-7 twist, chrome lined barrel. It makes me smile just to fondle it. It is definitely good to go! At a decent pace I can put PMC 55gr into 2ish MOA with a Millett DMS. I haven't heard any complaints about the other barrels either. I just had to drop the extra $150 because it was the last M4 upper they had at the Indy 1500 in Feb. Fit and finish is top notch and it is an almost perfect match with my DPMS lower.

Eta: My 1-9 twist barrel handles 77gr just fine. Its a 24" so it gets lots of spin by maximizing velocity.
tgad2  [Member]
10/22/2009 10:09:09 PM
Another thing worth mentioning is that the 1:9 will perform better than the 1:7 when using .22cal conversion kits using cheap ammo!
Molon  [Team Member]
10/22/2009 10:33:57 PM
Originally Posted By kcr121:

Originally Posted By walt_l:
BTW Colt 6920 comes in 1/9 and 1/7 both. I have both. Not sure if other twist rates are availble, but those tow for sure

Not true. If you have a Colt with a 1/9 twist barrel, it's a model other than the LE6920.



Yes it is true. Years ago the 6920s were sold with 1:9" twists. I personally owned two of them. After the 1:7" twist became popular and the 6920s were commonly being sold with the 1:7" twist, the model with the 1:9" twist was changed to the 6922.






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