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Posted: 12/15/2012 4:22:42 PM EDT


.....then why are so many barrels something less (1:8 or 1:9) ?

Are there advantages to a slower barrel
Link Posted: 12/15/2012 4:34:56 PM EDT
[#1]
you mean slower  


  bullet weights
Link Posted: 12/15/2012 4:51:14 PM EDT
[#2]
It's all about bullet weights.  The 1:7 was made the standard specifically for tracer ammo.  But 1:9 twist is actually ideal for stabilizing the XM193 55 grain round.  Some manufacturers compromised with a 1:8 twist.  

If you don't plan on ever shooting match weight bullets, or hunting rounds, can pretty much only shoot bulk 55 grain ammo, 1:9 twist is your best bet.
Link Posted: 12/15/2012 4:56:48 PM EDT
[#3]
Quoted:


.....then why are so many barrels something less (1:8 or 1:9) ?

Are there advantages to a slower barrel


"mil spec" doesnt mean this is how you have to make it.  thats just what the military is calling for on THEIR rifles.
Link Posted: 12/15/2012 5:16:24 PM EDT
[#4]
Quoted:


.....then why are so many barrels something less (1:8 or 1:9) ?

Are there advantages to a slower barrel


Different bullet weights and types require different rotational speed to stabilize the bullet in flight.
Link Posted: 12/15/2012 8:28:14 PM EDT
[#5]
Quoted:
It's all about bullet weights.  The 1:7 was made the standard specifically for tracer ammo.  But 1:9 twist is actually ideal for stabilizing the XM193 55 grain round.  Some manufacturers compromised with a 1:8 twist.  

If you don't plan on ever shooting match weight bullets, or hunting rounds, can pretty much only shoot bulk 55 grain ammo, 1:9 twist is your best bet.


Actually it's all about bullet length. Weight just kinda ties into that, since longer bullets typically mean they're heavier also.
1:7 was chosen to stabilize m856 tracers at very low temperatures... It's kind of a bonus that it will stabilize longer bullets such as today's 77 gr OTM. The m856 is only 64 gr, but it's also very long, thus the need for a faster twist.
Link Posted: 12/15/2012 9:09:29 PM EDT
[#6]
1:9 is about optimum for 62gr M855 rounds.  The military wanted to make sure their rifles would stabilize heavy (longer) tracer rounds even in arctic conditions so they mandated 1:7.
Link Posted: 12/15/2012 9:27:22 PM EDT
[#7]
Actually, this is the true story of the M16 and barrel twists.

First barrels were 1-14 which shot the 55 grain M193 round ok....except in very low temps.  In those cases, the bullets could have a tendency to tumble and keyhole.

As a result the military then switched to 1-12 twist, which stabalized the M193 projectiles well, even in arctic temperatures.    

When developing the M16A2 it was decided that M855 would be the issue ammo paired with the rifle.  It was also decided that they wanted the rifle to be able to fire a tracer that was capable of tracing out to 800 meters.  Thus, though they found 1-10 twist ideal for M855, they needed 1-7 twist to stabilize the length tracer they wanted to go with.  

In comes Bushmaster, who begins to make AR15 style clones after their original unique "Bushmaster Assault Rifle" design fizzles out.  Their original rifle design, as were 99% of the 5.56 rifles that were out there at the time, were 1-12 twist.  They decided to make their AR clones with a barrel twist that would do something unique in the commercial firearm's world....be able to stabilize the then unavailable current issue ammo, M855.  

They too found that 1-10 twist was the ideal twist for stabilizing the current issue ammo.  However, they decided to go one step further, for good measure, and settled on the then unheard of 1-9 twist (Bushmaster was the first to come up with this twist rate).

1-9 twist barrels could very accurately stabilize all of the available 5.56 and .223 ammo types at the time (except for the unavailable new issue 64 grain tracer).  This twist proved to be very popular for this reason, as most bullet types available for commercial sale were light varmint bullets, but the public liked the fact that they could fire any .223 round available at the time, esp the current issue round (much as with the appeal with 1-7 twist today, even though they may not shoot many 77 gr match rounds).  75 grain + match rounds were unheard of at the time, and most people viewed the 1-9 better than the 1-12 and the 1-7 (which was only offered by Colt) as the 1-9 could fire anything that the 1-7 twist could (except for the unavailable tracer round which no one cared about anyway), but could also fire the lighter varmint bullets more accurately.    

As a result, 1-9 became the dominant, and far preferred, twist until the mid 00's with the advent of the heavier long range match bullets.  

Nobody 10+ years ago preferred a 1-7 twist.
Link Posted: 12/15/2012 9:43:41 PM EDT
[#8]
Quoted:
It's all about bullet weights.  The 1:7 was made the standard specifically for tracer ammo.  But 1:9 twist is actually ideal for stabilizing the XM193 55 grain round.

Not even close.  1:9 is "ideal" for M855, or 68/69gr OTM types.  1:12 is ideal for M193.

Some manufacturers compromised with a 1:8 twist.  

Again, no.  1:8 is the best choice for the longest/heaviest bullets that are generally fired... 75, 77gr OTMs, even 80gr and various VLD match types that are too long to fit in a magazine.  It's the typical twist rate for service match and other long range target barrels.  The real "compromise" twist would be 1:8.5, which would be the minimum for the heaviest magazine-length factory ammo.  There used to be some 1:8.5 barrels made, before the advent of the long ~80gr match bullets for competition.

Quoted:
Are there advantages to a slower barrel

Yes.

Statistically better performance with poorer quality bullets.

Bullets follow correct trajectory to maximum range (tractability), which they can't do when spun to excessive stabilization.

If there are no downsides to excessive spin rate as some will claim, why do bulletmakers recommend slower twist rates?
Example:  http://www.bergerbullets.com/products/target-bullets/  Berger only recommends a 1:7 twist for their 90(!) grain VLD bullet.
Link Posted: 12/15/2012 10:47:14 PM EDT
[#9]



Quoted:



Quoted:

It's all about bullet weights.  The 1:7 was made the standard specifically for tracer ammo.  But 1:9 twist is actually ideal for stabilizing the XM193 55 grain round.  Some manufacturers compromised with a 1:8 twist.  



If you don't plan on ever shooting match weight bullets, or hunting rounds, can pretty much only shoot bulk 55 grain ammo, 1:9 twist is your best bet.




Actually it's all about bullet length. Weight just kinda ties into that, since longer bullets typically mean they're heavier also.

1:7 was chosen to stabilize m856 tracers at very low temperatures... It's kind of a bonus that it will stabilize longer bullets such as today's 77 gr OTM. The m856 is only 64 gr, but it's also very long, thus the need for a faster twist.
This. 1-8 would be ideal IMO.





 
Link Posted: 12/16/2012 8:37:18 AM EDT
[#10]
Quoted:
Actually, this is the true story of the M16 and barrel twists.

First barrels were 1-14 which shot the 55 grain M193 round ok....except in very low temps.  In those cases, the bullets could have a tendency to tumble and keyhole.

As a result the military then switched to 1-12 twist, which stabalized the M193 projectiles well, even in arctic temperatures.    

When developing the M16A2 it was decided that M855 would be the issue ammo paired with the rifle.  It was also decided that they wanted the rifle to be able to fire a tracer that was capable of tracing out to 800 meters.  Thus, though they found 1-10 twist ideal for M855, they needed 1-7 twist to stabilize the length tracer they wanted to go with.  

In comes Bushmaster, who begins to make AR15 style clones after their original unique "Bushmaster Assault Rifle" design fizzles out.  Their original rifle design, as were 99% of the 5.56 rifles that were out there at the time, were 1-12 twist.  They decided to make their AR clones with a barrel twist that would do something unique in the commercial firearm's world....be able to stabilize the then unavailable current issue ammo, M855.  

They too found that 1-10 twist was the ideal twist for stabilizing the current issue ammo.  However, they decided to go one step further, for good measure, and settled on the then unheard of 1-9 twist (Bushmaster was the first to come up with this twist rate).

1-9 twist barrels could very accurately stabilize all of the available 5.56 and .223 ammo types at the time (except for the unavailable new issue 64 grain tracer).  This twist proved to be very popular for this reason, as most bullet types available for commercial sale were light varmint bullets, but the public liked the fact that they could fire any .223 round available at the time, esp the current issue round (much as with the appeal with 1-7 twist today, even though they may not shoot many 77 gr match rounds).  75 grain + match rounds were unheard of at the time, and most people viewed the 1-9 better than the 1-12 and the 1-7 (which was only offered by Colt) as the 1-9 could fire anything that the 1-7 twist could (except for the unavailable tracer round which no one cared about anyway), but could also fire the lighter varmint bullets more accurately.    

As a result, 1-9 became the dominant, and far preferred, twist until the mid 00's with the advent of the heavier long range match bullets.  

Nobody 10+ years ago preferred a 1-7 twist.


Thanks,
and thanks
to Gamma762  



Link Posted: 12/16/2012 9:27:12 AM EDT
[#11]
the steyr aug had 1-9 twist back in 1977, maybe bushmaster copied them?
Link Posted: 12/16/2012 10:23:45 AM EDT
[#12]
I have 1:8 and 1:9 twists...
I don't have any 1:7 twist...
FWIW....
Link Posted: 12/16/2012 2:48:04 PM EDT
[#13]
Quoted:
Quoted:
It's all about bullet weights.  The 1:7 was made the standard specifically for tracer ammo.  But 1:9 twist is actually ideal for stabilizing the XM193 55 grain round.

Not even close.  1:9 is "ideal" for M855, or 68/69gr OTM types.  1:12 is ideal for M193.

Some manufacturers compromised with a 1:8 twist.  

Again, no.  1:8 is the best choice for the longest/heaviest bullets that are generally fired... 75, 77gr OTMs, even 80gr and various VLD match types that are too long to fit in a magazine.  It's the typical twist rate for service match and other long range target barrels.  The real "compromise" twist would be 1:8.5, which would be the minimum for the heaviest magazine-length factory ammo.  There used to be some 1:8.5 barrels made, before the advent of the long ~80gr match bullets for competition.

Quoted:
Are there advantages to a slower barrel

Yes.

Statistically better performance with poorer quality bullets.

Bullets follow correct trajectory to maximum range (tractability), which they can't do when spun to excessive stabilization.

If there are no downsides to excessive spin rate as some will claim, why do bulletmakers recommend slower twist rates?
Example:  http://www.bergerbullets.com/products/target-bullets/  Berger only recommends a 1:7 twist for their 90(!) grain VLD bullet.


I think you have it backwards. A 1:7 twist is a Faster spin than 1:9.  The twist rate is measured by how many inches of the barrel for the rifling to make one complete revolution; hence a 1:7 imparts one complete revolution in 7 inches of barrel.....a 1:9 uses 9 inches of barrel.
Heavier bullets require more spin to stabilize in flight, that is why Berger recommends the 1:7 for their 90gr. VLD.
Link Posted: 12/16/2012 2:56:07 PM EDT
[#14]
Quoted:
I think you have it backwards. A 1:7 twist is a Faster spin than 1:9.  The twist rate is measured by how many inches of the barrel for the rifling to make one complete revolution; hence a 1:7 imparts one complete revolution in 7 inches of barrel.....a 1:9 uses 9 inches of barrel.
Heavier bullets require more spin to stabilize in flight, that is why Berger recommends the 1:7 for their 90gr. VLD.

I'm not backwards at all.

Bullet manufacturers generally recommend the twist that will properly stabilize the bullet in question, without excessive rotational speed/stability which negatively impacts long range performance.
Link Posted: 12/16/2012 2:56:57 PM EDT
[#15]
2rnd burst
Link Posted: 12/16/2012 3:31:14 PM EDT
[#16]
Quoted:
Quoted:
I think you have it backwards. A 1:7 twist is a Faster spin than 1:9.  The twist rate is measured by how many inches of the barrel for the rifling to make one complete revolution; hence a 1:7 imparts one complete revolution in 7 inches of barrel.....a 1:9 uses 9 inches of barrel.
Heavier bullets require more spin to stabilize in flight, that is why Berger recommends the 1:7 for their 90gr. VLD.

I'm not backwards at all.

Bullet manufacturers generally recommend the twist that will properly stabilize the bullet in question, without excessive rotational speed/stability which negatively impacts long range performance.


I think we're on the same page of understanding twist rates....I think it was the wording of your example that made it sound reversed.
Link Posted: 12/16/2012 4:05:38 PM EDT
[#17]
Quoted:
Quoted:
It's all about bullet weights.  The 1:7 was made the standard specifically for tracer ammo.  But 1:9 twist is actually ideal for stabilizing the XM193 55 grain round.

Not even close.  1:9 is "ideal" for M855, or 68/69gr OTM types.  1:12 is ideal for M193.

Some manufacturers compromised with a 1:8 twist.  

Again, no.  1:8 is the best choice for the longest/heaviest bullets that are generally fired... 75, 77gr OTMs, even 80gr and various VLD match types that are too long to fit in a magazine.  It's the typical twist rate for service match and other long range target barrels.  The real "compromise" twist would be 1:8.5, which would be the minimum for the heaviest magazine-length factory ammo.  There used to be some 1:8.5 barrels made, before the advent of the long ~80gr match bullets for competition.

Quoted:
Are there advantages to a slower barrel

Yes.

Statistically better performance with poorer quality bullets.

Bullets follow correct trajectory to maximum range (tractability), which they can't do when spun to excessive stabilization.

If there are no downsides to excessive spin rate as some will claim, why do bulletmakers recommend slower twist rates?
Example:  http://www.bergerbullets.com/products/target-bullets/  Berger only recommends a 1:7 twist for their 90(!) grain VLD bullet.


+100
Link Posted: 12/16/2012 6:29:19 PM EDT
[#18]
I think it's funny how people are shouting "x twist rate is the best for y bullet weight". The optimal twist rate depends entirely on the recommended rpm of the bullet and the muzzle velocity. Before taking in account outside forces of course.

Barrel twist does not induce a static rpm on the bullet that is independent of the muzzle velocity.
Link Posted: 12/16/2012 8:02:10 PM EDT
[#19]
Quoted:
I think it's funny how people are shouting "x twist rate is the best for y bullet weight". The optimal twist rate depends entirely on the recommended rpm of the bullet and the muzzle velocity. Before taking in account outside forces of course.

Barrel twist does not induce a static rpm on the bullet that is independent of the muzzle velocity.

I think in the context of this discussion, we're generally on the same page that we're talking about the 223/5.56 cartridge within the range of barrel lengths typical for the AR15.
Link Posted: 12/16/2012 8:18:48 PM EDT
[#20]
Quoted:
I think in the context of this discussion, we're generally on the same page that we're talking about the 223/5.56 cartridge within the range of barrel lengths typical for the AR15.

Muzzle velocity difference between a 16" and a 20" is gonna be somewhere around 150fps for most rounds. That's a rough a number btw, I'm just making the point. If you want to throw in extremes like a varmint 24" barrel and a sbr 10.5" you'll be talking about quite difffernet muzzle velocities and optimal twist rates for the same bullet.

Link Posted: 12/17/2012 1:49:50 PM EDT
[#21]
Shooting light weight (<50 grain), thin skinned varmint bullets at high velocities out of a 1 in 7 barrel will sometimes tear the bullet apart due to centrifugal forces.  I find that 1 in 9" is a good all-around compromise.
Link Posted: 1/15/2013 2:05:48 PM EDT
[#22]
Quoted:
I think it's funny how people are shouting "x twist rate is the best for y bullet weight". The optimal twist rate depends entirely on the recommended rpm of the bullet and the muzzle velocity. Before taking in account outside forces of course.

Barrel twist does not induce a static rpm on the bullet that is independent of the muzzle velocity.


Link Posted: 1/17/2013 7:20:57 AM EDT
[#23]
So if I have a 16 inch barrel and I wish to shoot 55-65 grain ammo what would be the best twist to be able to shoot at long ranges such as 500 yards?
Link Posted: 1/17/2013 7:26:33 AM EDT
[#24]
Quoted:
So if I have a 16 inch barrel and I wish to shoot 55-65 grain ammo what would be the best twist to be able to shoot at long ranges such as 500 yards?


A gain twist from 1-14 to 1-6.5.
Link Posted: 1/17/2013 10:45:25 AM EDT
[#25]
Typed this out in another of the current twist rate threads:
http://www.ar15.com/forums/t_3_118/599029_twist_rate_help.html&page=2#i5811794


Quoted:
Quoted:
So if I have a 16 inch barrel and I wish to shoot 55-65 grain ammo what would be the best twist to be able to shoot at long ranges such as 500 yards?

A gain twist from 1-14 to 1-6.5.

Hope you're being sarcastic.

For the uninformed, a gain twist is something that is used in large caliber heavy weapons to reduces stresses on both the weapon (& weapon platform) as well as the projectile, its fusing etc.  Gain twist has been experimented with and found to not work in small arms... one major reason being the relatively long bearing surfaces of small arms projectiles in the bore versus the driving bands of the large caliber ammo.

Quoted:
They too found that 1-10 twist was the ideal twist for stabilizing the current issue ammo.  However, they decided to go one step further, for good measure, and settled on the then unheard of 1-9 twist (Bushmaster was the first to come up with this twist rate).

Missed this earlier.  Bushmaster didn't invent the 1:9 twist, in fact 1:9 was the original twist of the M16A2 development program along with the M855 ammo.  Never heard of 1:10 being intended for M855 either, from what I know the 1:10 was for the 62 to 65gr lead core type ammo, of which there are a variety of hunting-oriented bullet types.
Link Posted: 1/17/2013 12:09:24 PM EDT
[#26]
Both 1/7 and 1/9 shoot 55 gr fmj, but 1/7 will shoot the heavier bullets to. For me it was 1/7
Link Posted: 1/17/2013 2:48:36 PM EDT
[#27]
I have 3 rifles , a 18" 1x7, 18" 1x9 and a 16" 1x9. Not a dimes worth of difference at 100 yds.with 55 grain ball or 55 gr vmax. 68 gr does a little out of the 1x7 but not a lot. I would get what I could get a deal on and call it good.
Link Posted: 1/17/2013 6:39:08 PM EDT
[#28]
I believe the S&W M&P15 Sport has a 1/8" gain from 1:8 to 1:7.



My M&P 15 is 1:9 only.
Link Posted: 1/20/2013 12:46:31 PM EDT
[#29]
At the time the military adopted the 1:7” twist it was supposedly to stabilize M856 tracer.
I never saw it issued except in belts, but then we did not have any LMGs that could be mag fed either.
Civilians have many choices for ammo.
Over stabilization can magnify imperfections in bullets.
Link Posted: 1/20/2013 3:08:14 PM EDT
[#30]
Quoted:
At the time the military adopted the 1:7” twist it was supposedly to stabilize M856 tracer.
I never saw it issued except in belts, but then we did not have any LMGs that could be mag fed either.
Civilians have many choices for ammo.
Over stabilization can magnify imperfections in bullets.


The standard issue for UBL is 170 rounds M855 and 40 rounds M856.  

This is not to say this is what and/or all that gets carried, or in what combination - but it is the standard load.

Needless to say, for BRM or qualification, no M856 is issued, however loose (boxed, really) M856 is most certainly issued in quantity.  

~Augee
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