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Old 09-25-2004, 09:44 AM   #1
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1/7 and 1/9 twist Difference

A couple of rookie question everyone.
The difference between a 1/7 and 1/9 twist barrel. What the advantage of one over the other?
Secondly, in order for a 14.5" barrel to be NFA legal, does the Phantom FS need to be permanently installed as in welded? Can the FS be purchased and screwed on?

Thanks!!!
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Old 09-25-2004, 10:33 AM   #2
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The rifling rate determines the length of bullet that the rifle can stabilize. All things being equal, a heavier bullet is longer, and therefore requires faster rifling to stabilize it. Tracers, being partly made of tracing material rather than lead, are less dense and are therefore longer for a given weight. When the rifling isn't fast enough, a round starts to wobble in flight and accuracy drops off dramatically. (think of throwing a football with a tight spiral vs. a throw that's all wobbly.) There can also be small decreases in accuracy when the rifling rate is too high. The ideal is to have just enough twist to stabilize your bullet.

The US army started out using m16 barrels that were 1:14. (i.e. one twist for every 14inches of barrel) They found that under arctic conditions the air was very dense, and sometimes the 1:14 barrels wouldn't stabilize 55 grain tracers. The army then changed to 1:12 rifling. Later on they switched over to the new 62 grain bullets. To stabilize a 62 grain tracer reliably, they had to change their rifling rate to 1:7. When the manufacturers make AR's for the civilian market, they usually use 1:9 barrels. It's fast enough to stabilize 62 gr ball, it's not as far off for 55 gr ball, and they figure you're not likely to be firing 62 gr tracers. Bottom line, if you want to shoot 62gr tracers, get a 1:7. Otherwise, 1:9 is fine. If you're not going to shoot tracers or 62 gr ammo, 1:14 would even work.
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Old 09-25-2004, 10:33 AM   #3
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the 1/7 is necessary to stabilize tracers (M856, I think?).

if you do not shoot tracers, 1/9 is perfectly acceptable for mil ball.

if you shoot moire than about 69 or so grain bullets, you need more than 1/9.

1/7 will wear out a little faster than will 1/9, but none of us will likely ever see the difference.

given the choice, I will take 1/7, even though tracers are forbidden to me. Given no choice, I will take the 1/9 and be happy and never really see the difference.

unless you register it as a SBR, yes, something must be permanently attached to the barrel to make it over 16" long.
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Old 09-25-2004, 12:23 PM   #4
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The 1:9 twist is a commercial compromise to provide for a wider range of bullet weights. For me anyway, it has done well with bullet weights from 55 to 69 grains.

The 1:7 will shoot the 55 grain stuff ok but not the greatest. It will shoot the 69 - 80 grain stuff great. For this, personally, I like the 1:7.
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Old 09-25-2004, 02:05 PM   #5
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The real world answer for the tactical shooter:
1 in 7 with 55gr and lighter bullets destabilizes the round and does ruthless internal damage to human targets. Not cool for grouping for competition or tactical longrange intervention shooting as trajectory is often unpredictable at these distances with standard combat ammunition... especially since the ss109 was backed off to 62gr from 69.
1 in 9 with about anything. Bushmaster settled on this twist as it stabilizes all but the heaviest competition longrange rounds. I've grouped well with everything from 40gr vmax to ss109 rounds without many keyholes. 16" bbl.
1 in 8 has been standardized for the long range competition rifles with 70+ grain rnds. and exceptionally "hot" rounds. Usually to be had in varminter length barrels. 20" - 26"

These barrels are the standard and can be had very quickly from numerous sources set up for the AR15. Anything else, will probably have to custom ordered.
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Old 09-25-2004, 05:43 PM   #6
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Quote:
Originally Posted by 108DRAGON
The real world answer for the tactical shooter:
1 in 7 with 55gr and lighter bullets destabilizes the round and does ruthless internal damage to human targets. Not cool for grouping for competition or tactical longrange intervention shooting as trajectory is often unpredictable at these distances with standard combat ammunition...
Disagree. It is physically imposible for a projectile to be destabilized by spinning faster than it needs to.

You can parade all the "evidence" you want, but whatever tumbling might have occurred did not happen in flight. I can guarantee you that.

SLOW twists that barely stabilize a bullet (like 1 in 12 for 55 gr 223) will allow tumbling to occur much more easily once the bullet hits flesh.

Bottom line is this: lighter bullets do NOT overstabilize or understablize in faster than necessary barrels. SOME think jacketed bullets MIGHT come apart in flight if spun too much. Does not happen consistently and shooting light bullets fast in fast twists is not a guarantee for blowing them up.

My rifle (with a 1 in 8 barrel) shoots ammo from 45 to 75 grains just about equally well.
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Old 09-25-2004, 05:58 PM   #7
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i have to agree with SWO daddy...

my colt 1:7 barrel will shoot 5 52g SMKs into a dime off the bench. forget about me holding it, im not that steady. those holes by the way are nearly perfectly round...

100yd distance with a scope by the way...

david
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Old 09-25-2004, 10:35 PM   #8
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Quote:
Originally Posted by swo daddy
Quote:
Originally Posted by 108DRAGON
The real world answer for the tactical shooter:
1 in 7 with 55gr and lighter bullets destabilizes the round and does ruthless internal damage to human targets. Not cool for grouping for competition or tactical longrange intervention shooting as trajectory is often unpredictable at these distances with standard combat ammunition...
Disagree. It is physically imposible for a projectile to be destabilized by spinning faster than it needs to.

You can parade all the "evidence" you want, but whatever tumbling might have occurred did not happen in flight. I can guarantee you that.

SLOW twists that barely stabilize a bullet (like 1 in 12 for 55 gr 223) will allow tumbling to occur much more easily once the bullet hits flesh.

Bottom line is this: lighter bullets do NOT overstabilize or understablize in faster than necessary barrels. SOME think jacketed bullets MIGHT come apart in flight if spun too much. Does not happen consistently and shooting light bullets fast in fast twists is not a guarantee for blowing them up.

My rifle (with a 1 in 8 barrel) shoots ammo from 45 to 75 grains just about equally well.
not just that, but the barrel twist has noting to do with "tumbling" in flesh. ALL pointed bullets will seek to turn 180 degrees in flesh. it's physics. AND - the "ruthless" internal damage (normally, though mistakenly attributed to slower twist rates) is actually believed to be a function of linear velocity, jacket integrity and impact medium density. The M193 was a fortuitous combination that produced fragmenting at the cannelure during the inevitable yaw through flesh. the detached fragments compromised the elastic potential of the temporary wound cavity walls, thus creating more severe results.

or so says this Army surgeon who cut on many such wounds in Vietnam.
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Old 09-26-2004, 02:02 AM   #9
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I figured I'd get that reaction ... How long are your 1 in 7 barrels? Are they military chrome lined jobs, or tight match bores? What makes anyone think that a 3700fps projectile would be stabilized in 16" of even a 1 in 5 twist barrel? Why did the military go to 1 in 9 citing better accuracy with the same military 55gr. they've always used? The ss109 has only been a fairly recent developement in the lifespan of the .223. The 1 in 9 twist barrels in current issue weapons are not producing the same terminal balistics results that the 1 in 7 twist Vietman era barrels did.
There are enough complaints about .223 "passing right through a tango in multiple hits but not dropping him" out of Afghanistan and Iraq right now to fuel serious debate about reenstating the .308 M14 or putting the 6.8 into play. The only difference between Vietnam and now is the short 1 in 9 barrel. No keyholing, no 180 action.. quote "in one side, out the other". Just like Itdave's targets.
I am fully versed in terminal ballistics & tissue disruption theory. If the bullet is doing the "football flight" thing as a result of what happens between the time you pull the trigger and the point of impact into the target, and the only difference is the barrel:
A: If you take away everything that absolutely cannot be, whatever is left, nomatter how improbable, is the truth. Its got to be the barrel.
B: If the bullet is at a yaw upon contact with the target, delloro's physics will cause it to continue in the direction of its axis when it entered flesh. Kinda like what happens when you drop a coin into a bucket of water.
Its cool that the accidental nature of the bullets design lends heavily to the result. Truth be known, it may have even been hasty manufacturing and loose tolerances in those earlier barrels.
I didn't really mean to p*** any of you off. I realize that every one has thier "pet" combo.. I do too. And everyone has thier own opinion.. as do I. Please don't get all sad at me, guys. If it helps, o.k., you guys are absolutely right.
Incidently, Itdave, I print like that with 16" 1 in 9. On a good day, I can do moa with a flyer or two without the U.S. Optics SN-4.
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Old 09-26-2004, 05:47 AM   #10
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Quote:
Originally Posted by 108DRAGON
I figured I'd get that reaction ... How long are your 1 in 7 barrels? Are they military chrome lined jobs, or tight match bores? What makes anyone think that a 3700fps projectile would be stabilized in 16" of even a 1 in 5 twist barrel? Why did the military go to 1 in 9 citing better accuracy with the same military 55gr. they've always used? The ss109 has only been a fairly recent developement in the lifespan of the .223. The 1 in 9 twist barrels in current issue weapons are not producing the same terminal balistics results that the 1 in 7 twist Vietman era barrels did.
There are enough complaints about .223 "passing right through a tango in multiple hits but not dropping him" out of Afghanistan and Iraq right now to fuel serious debate about reenstating the .308 M14 or putting the 6.8 into play. The only difference between Vietnam and now is the short 1 in 9 barrel. No keyholing, no 180 action.. quote "in one side, out the other". Just like Itdave's targets.
I am fully versed in terminal ballistics & tissue disruption theory. If the bullet is doing the "football flight" thing as a result of what happens between the time you pull the trigger and the point of impact into the target, and the only difference is the barrel:
A: If you take away everything that absolutely cannot be, whatever is left, nomatter how improbable, is the truth. Its got to be the barrel.
B: If the bullet is at a yaw upon contact with the target, delloro's physics will cause it to continue in the direction of its axis when it entered flesh. Kinda like what happens when you drop a coin into a bucket of water.
Its cool that the accidental nature of the bullets design lends heavily to the result. Truth be known, it may have even been hasty manufacturing and loose tolerances in those earlier barrels.
I didn't really mean to p*** any of you off. I realize that every one has thier "pet" combo.. I do too. And everyone has thier own opinion.. as do I. Please don't get all sad at me, guys. If it helps, o.k., you guys are absolutely right.
Incidently, Itdave, I print like that with 16" 1 in 9. On a good day, I can do moa with a flyer or two without the U.S. Optics SN-4.

Your incorrect statements in order.
1. The military uses 1in7 not 1in9 as you state.
2.Vietnam issue barrels(and all M16A1's) are 1in12 not 1in7
3.The problem with the current ammo is the LENGTH of the barrel it is being fired from,not the TWIST RATE. WE are not getting the velocity we need for reliable bullet fragmentation. That is a function of barrel length.
4.SS109/M855 has been around for about 20 years now

5.As a reply to your entire last paragraph. We cut the barrels back from 20" to 14" and changed the impact velocity. THAT is the reason we are not getting the perfomance from the M193/M855 we should be. If the only thing that was changed was the twist rate I would agree it had to be that. But something much more significant WAS changed. The barrel length. Cut your pet 300 mag down to a 18" barrel and you will suddenly have a really loud 30/30. Try to whack an elk from 500 yards and you will see a difference in stopping power,if you even hit it. Velocity is the squaring function in the stopping power equation. With ANY highpower rifle changing it will have more effect on bullet perfomance than ANY other factor(bullet weight,shape,or TWIST RATE) T.J. Robertson
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Old 09-26-2004, 06:46 AM   #11
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I agree with trobertson5-0. Except the very early M16s had a 1-14 twist barrel. The reason I was told for the change to a 1-12 twist was the need for better long range accuticy,300 meters. I just spent time, yesterday, with an individual who just got back from Afaganistan. He said the M4s work fine out to about 250 or so meters. Each squad is now being issued 2 M14s for use over 250 meters. I have read after action statements that there was a whole mag of rounds put into the guy chest and he still kept going. When the body is checked he has one hit. This goes back to Vietnam.
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Old 09-26-2004, 08:20 AM   #12
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Quote:
Originally Posted by Gunseller
I have read after action statements that there was a whole mag of rounds put into the guy chest and he still kept going. When the body is checked he has one hit. This goes back to Vietnam.
A direct result of when you don't allow your general military (non spec-ops) to get sufficient "live fire" practice. Blame Clinton for this one too.

Ever wonder why there's such a proliferation of ACOGs for the front line (isn't that an oxymoron anymore?) troops today?

BTW, I thought the switch from 1/14 to 1/12 was because the early M16 variants didn't fair too well in Arctic testing? Wait a minute. Never mind. That would be an "accuracy" change! Duh!

Moe
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Old 09-26-2004, 12:34 PM   #13
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Mjmensale you are correct about the reason for the switch from 1in14 to 1in12. In conditions below freezing the 1in14 would not stabilize the 55gr tracer. It was longer than the 55gr. ball and harder to stabilize. That is the only reason I have heard for the switch but longer range accuracy could have been a factor as well. T.J. Robertson
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Old 09-26-2004, 04:15 PM   #14
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I'm sorry, but how can a bullet be "too stable"? I hear the word
"overstabilized" and I just cringe. I think the bullet can be "overspun" if that is the correct grammar and/or spelling.
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Old 09-26-2004, 04:47 PM   #15
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If a bullet is spun faster than is needed the imperfections in the bullet will show as an imbalance and result in a loss in accuricy. The reason that I was first told and read about for a change in twist in a M16 from 1-14 to 1-12 was 1-14 could not consistantly hit a USSR helmet at 300 meters.
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Old 09-26-2004, 05:13 PM   #16
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For whatever it's worth, I'm currently shooting a 1/9 barrel and overall it performs well with bullets up to about 69gr. After that it gets a little dicey as far as accuracy goes. Personally, I don't plan on shooting 80gr bullets one at a time in high power matches so I doubt I'll ever need a 1/7 twist barrel. It also seems to me that I might not get the velocities I want out of a 1/7 barrel with "normal" weight bullets as the more agressive twist would attempt to slow the bullet down more than I would like. When I shoot out my 1/9 I'm going with 1/8 for everything.
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Old 09-26-2004, 07:43 PM   #17
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I have two Mil issue 1 in 7 barrels, Vietnam era. I think they are post '70. They have the triangular hand guards. I have not bothered using them as the bores aren't in the best shape.. but they are issue barrels. They came from Ft. Riley.
Again, everyone seems to have different opinions. I stand down on mine, as apparently, there are at least three different twist barrels issued. And come to find out, some Special Forces even got shorter than 20" barrels back then.
Again, sorry if I pissed anyone off.
If we must, please accept Southern Colorado hospitality and join me for a few days here, I'll have 1 in 7, 1 in 8, 1 in 9 ( I have the previous), and whatever else I can dig up mounted on some uppers. We'll test them all objectively. If I'm wrong, I'll say so on this site.. and to each of you in person. -is that honorable enough?
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Old 09-27-2004, 08:03 AM   #18
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Quote:
Originally Posted by SP Tiger
I'm sorry, but how can a bullet be "too stable"? I hear the word
"overstabilized" and I just cringe. I think the bullet can be "overspun" if that is the correct grammar and/or spelling.
IIRC the "overstabilized" thing has to do with a bullet, fired at an angle, not "nosing over" and thus not hitting the target straight on. IOW, the rotational axis and the line of flight are not coincident. I know that's one concern, but i do not recall the validity of that concern.
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Old 09-27-2004, 08:09 AM   #19
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Quote:
Originally Posted by 108DRAGON
...-is that honorable enough?
What? Nobody is impugning your honor! heck, we (some of us) just think you have the twist-rate thing a bit backwards. No big deal, it's not like we're debating SA, Inc. Garands or anything....

But thanks for the generous invitation, I can't make it anytime soon, but if you are ever here in So. Cal. we can shoot what I have and figure out the technical stuff over some beer.
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Old 09-27-2004, 10:09 AM   #20
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Quote:
Originally Posted by 108DRAGON
I have two Mil issue 1 in 7 barrels, Vietnam era.
See now, you keep making it harder and harder to believe anything you say.

1 in 7 barrels did not exist in M16s until the M16A2, which issued to the troops circa 1982. That's what, 9 years after Vietnam was officially over?

You need to get your facts straight if you want to be taken seriously when talking about technical matters.
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