View Full Version : 1:7 vs 1:9
This is a rookie question I am sure, but what are the advantages and disadvantages if a 1:7 twist versus a 1:9 twist? Is one preferable to another?
spamsammich
05-03-10, 21:19
covered many times. search via google or read the knowledge base threads stickied at the top of the technical forum.
also, read the descriptions at this link
http://spreadsheets.google.com/pub?key=pwswheghNQsEuEhjFwPrgTA&single=true&gid=5&output=html
The advantage of the 1:7 twist is that it lets you run a longer bullet at lower velocities. The disadvantage is that it can put too much RPM on a lighter bullet that's pushed to higher velocities.
Longer bullets require more RPM to stabilize than shorter bullets regardless of weight. With conventional bullet construction, the heavier the bullet, the longer; the lighter, the shorter.
(Bullets made from lighter materials or of the VLD [Very Low Drag] profile will be longer than conventional bullets of the same weight but that's not of much concern to the average AR driver.)
Bullets need to be spun a minimum RPM to be stable in flight. RPM is determined by velocity and twist rate. All else being equal, a longer barrel, up to a certain point, will produce more velocity than a shorter barrel. That's why a 1:9 twist in a 14.5" barrel may not stabilize a 69 gr bullet, but will in a 20".
Before we go further, we must be clear about one thing: You cannot over-stabilize a bullet, no matter how tight the twist. A bullet is either stable or it is not. If the RPM imparted to a bullet causes it to wobble, it isn't overstable, it is unstable.
The biggest problem with a bullet at too high of an RPM is it's construction. It's possible to spin a bullet so fast that it comes apart from the centrifugal forces. Bullets on the verge of destruction will leak lead dust in flight. This is a problem driving 45 & 55 gr bullets from your AR from a 20" barrel with a 1:7. It's not as much of a problem from carbine length barrels due lower velocities.
It's also possible to drive a bullet so fast in a tight twist barrel that the rifling strips the jacket.
Also, for all our technology, no two barrels are cut with the exact same twist. A barrel with cut rifling can easily vary a half inch either way. Barrels cut to be 1:7 can vary from 1:6.5 - 1:7.5. (I don't think this is a problem with hammer forged barrels as they are formed around a mandrel.)
To figure out which twist rate is best for you is to decide what bullet you'll be shooting at what velocity. I don't know if anyone has data to give us an idea what bullet lengths are stable at what RPM. If such data does exist, it would help. Even so, one cannot be 100% sure until shots are fired. Some run 55 gr bullets in short barrels with the 1:7 twist just fine. Others find that it gives poor results.
In general, the 1:7 should give you good accuracy with 69 & 55 gr bullets in the short carbines, and 69 - 77 gr and VLD bullets with a 20" rifle. However, that is a general statement and there are folks whose experience differs
That's why my latest barrel is a 1/8.
arizonaranchman
05-03-10, 22:06
The 1:7 is more all-purpose if you shoot a variety of bullet weights, especially the heavier/longer weights.
If all you shoot is budget/bulk 55 or 62 grain you'll be just fine with a 1:9 twist.
I have rifles in both and haven't had any trouble with the 55-62 gr bullets that I use. I reload and shoot bulk FMJ's of the lighter weight so not too crucial in my case.
Thanks for all the info. My DDXV is a 1:7, and hasn't given me any trouble. I just have noticed that some have a 1:7 and some have a 1:9 and wondered what the difference was.
Dumb question: Are there any 1/9 barrels that are made of milspec 4150 that are HP/MPI?
I'm wondering if there is a reason to look beyond 1/9 besides the twist.
Having both 1 in 9" twist and 1 in 7" I would strongly suggest following the consensus here to purchase a 1 in 7".
My 7" twist rifle have no problems shooting weights from 45-77 grains.
Cameron
Who needs an account on Barfcom?
Barfcom is here!!
spamsammich
05-04-10, 02:22
been spending more and more time on LF myself ;)
ChicagoTex
05-04-10, 03:44
Are there any 1/9 barrels that are made of milspec 4150 that are HP/MPI?
In terms of factory barrels from AR manufacturers, not that I know of. Semi-custom 1/9" barrels from outfits like AR15barrels, WOA, and ADCO can be ordered, however.
In terms of factory barrels from AR manufacturers, not that I know of. Semi-custom 1/9" barrels from outfits like AR15barrels, WOA, and ADCO can be ordered, however.
That's what I thought.
So if we want to talk about 1/9 4140 CM steel barrels, maybe we should go to ARFCOM instead, huh?
:D
Belmont31R
05-04-10, 14:16
That's what I thought.
So if we want to talk about 1/9 4140 CM steel barrels, maybe we should go to ARFCOM instead, huh?
:D
Yeah.....this question gets posted over there at least 5x a day, and then you get the plethora of people expounding on good their junk 1/9 barrel is with the whole 200rds they have through it.
Also, for all our technology, no two barrels are cut with the exact same twist. A barrel with cut rifling can easily vary a half inch either way. Barrels cut to be 1:7 can vary from 1:6.5 - 1:7.5. (I don't think this is a problem with hammer forged barrels as they are formed around a mandrel.)
this is the only thing worthwhile in your reply as it explains why some 1/9 twist barrels can shoot 75gr bullets and others can't...
i've been shooting 45-55gr bullets through my 1/7 twist barrels for years... one of the only useful threads on TOS showed a guy shooting 45gr VMAX through his 1/7 in 1MOA group... 69-77gr have had much better accuracy for me, but I think that's more a factor of bullet design (69/77gr SMK and 75gr hornady OTM) as opposed to weight and twist since the 45gr were winchester white box (JHP advertised as 3600fps) and the 55gr have all been federal XM193...
i've never had jacket separation or any of the other nonsense myths perpetuated by low end barrel manufacturers to peddle their 1/9 twist barrels on everyone...
My 1/9's have always done fine with the ammunition I shoot. But just out of curiosity, why would a manufacturer produce 1/9 instead of 1/7 if the 1/7 is the "correct" barrel twist? It would cost them any more. Why wouldn't they have started out producing 1/7's?
Most range guys will shoot 45 gr. which does fine with a 1/9 - 75 gr. depending on the particular barrel. 55 gr. is probably still the most popular.
It was my understanding that the 1/7 was developed by the Military to stabilize longer tracer rounds, and heavier rounds.
Why would 1/9 be junk pushed by low end barrel manufacturers?
Dumb question: Are there any 1/9 barrels that are made of milspec 4150 that are HP/MPI?
I'm wondering if there is a reason to look beyond 1/9 besides the twist.
I didn't see any barrel specs, but the Colt AR6721 AR-15A3 has a 1:9 barrel.
http://www.colt.com/law/ar15a3.asp
RogerinTPA
05-04-10, 15:45
My 1/9's have always done fine with the ammunition I shoot. But just out of curiosity, why would a manufacturer produce 1/9 instead of 1/7 if the 1/7 is the "correct" barrel twist? It would cost them any more. Why wouldn't they have started out producing 1/7's?
Most range guys will shoot 45 gr. which does fine with a 1/9 - 75 gr. depending on the particular barrel. 55 gr. is probably still the most popular.
It was my understanding that the 1/7 was developed by the Military to stabilize longer tracer rounds, and heavier rounds.
Why would 1/9 be junk pushed by low end barrel manufacturers?
Most AR Manufacturers like BM, OLY, Stag, etc.... cater to the general public, who are in fact "collectors" and Not Users. They also Count on the ignorance of the General Public, in their marketing to boost sales and build them accordingly. That's why their ARs don't rank well or are not present on the "chart".
Most will never shoot there ARs enough or try different bullet weights, to know the difference.
The average AR collector will be lucky to shoot their weapon twice a year and less that 200 rounds, maybe.
Our sport (Training and proficiency) on this forum, is way too expensive for the average AR owner, to maintain any level of proficiency. (I average 12K to 14K rounds fired per year, 2/3 of that through my ARs and I'm probably at the bottom of some members ammo expenditures on this forum).
Most of the "collectors" will never shoot anything above a 55 grain, maybe, 62 grain, if that, due to cost.
The overwhelming majority will never take a formal class from an accredited instructor or school, nor seek out additional training beyond reading the operator's manual, an unqualified friend, bubba, or a self proclaimed expert's opinion.
As far a barrel steel, over 85% of AR owners could care less and wouldn't know the difference, including LEOs and former Mil types.
A 1x9 ARs suits the overwhelming majority of AR collectors, except for the enthusiast on this forum.
Well I read the above post and still do not understand the answer except the 1/7 really is not needed in civilian or L.E.O. applications.
And for L.E.O shooting distances and grain that we shoot, seems 1/9 is fine. So unless you are military, insist on mil-spec, or have a need such as heavy hunting rounds, 1/9 sounds fine.
Belmont31R
05-04-10, 18:14
Well I read the above post and still do not understand the answer except the 1/7 really is not needed in civilian or L.E.O. applications.
And for L.E.O shooting distances and grain that we shoot, seems 1/9 is fine. So unless you are military, insist on mil-spec, or have a need such as heavy hunting rounds, 1/9 sounds fine.
No.
If you put light round counts and never shoot ammo over 70GR 1x9, and a cheaper gun will probably be just fine.
If you shoot a lot, and shoot ammo over 70 grains (almost all match ammo, and the better go to ammo loads) then 1x7 is a must, and the associated general higher quality.
When a 1x7 shoots everything from 55 to 77 grain ammo just fine the only reason to go 1x9 is because you are buying a low cost upper which is usually associated with cheaper uppers and barrels.
https://www.m4carbine.net/showthread.php?t=11829
https://www.m4carbine.net/showthread.php?t=45272
https://www.m4carbine.net/showthread.php?t=48753
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