Will a 1 in 8 twist barel shoot 55 Gr. FMJ bullets OK or would I need to go to at least 60 or 62 Gr. bullets. If not, what weights will it shoot. Also what bullet weights are best for 1 in 9 twist. The 1 in 9 twist Bushmaster I used to have seemed to shoot 55 Gr. FMJ's reasonably well, not great though. Thanks, AC
AR barrel twist Q
- Thread starter Walkalong
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HorseSoldier
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1:8 twist will be fine for 55 grain.
This has been beaten to death. 1:9 will shoot M193 and M855 equally well. Faster twists favor heavier bullets, and may have dispersion issues with lighter bullets, but you certainly can shoot lighter bullets with a faster twist. If accuracy with heavy bullets is important, pick 1:8 or 1:7.
Let's beat it a little further...some of us are relative newbies to the AR or rifles in general.
Other than the possibility of fragmenting very light skinned, lightweight bullets, is there an accuracy downside to "overstabilizing" a 223 round? Say the 55 FMJ since they're so common.
/Bryan
Other than the possibility of fragmenting very light skinned, lightweight bullets, is there an accuracy downside to "overstabilizing" a 223 round? Say the 55 FMJ since they're so common.
/Bryan
DMK
Member
That's actually a very good question.
I'd think that the gyroscopic effect would still keep it true (spin a bicycle wheel faster and it still doesn't want to turn), but for some reason, both my 1/7 twist rifles (one 16" and one 20") shoot bigger groups with 55grs than my 1/9 twist rifles do (again, one 16" and one 20"). The 1/7s do shoot 75grs more accurately than the 1/9s though.
Then again, perhaps it's just due to my particular barrels' personalities. I suppose every mass produced barrel has an individual preference for a certain bullet and smaller bore barrels seem especially finicky.
Jim Watson
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A tough bullet like the military ball FMJ will not be hurt by a faster than necssary twist and may be helped by it. The first use of fast twist barrels - well, back then they thought 10" was a fast twist - was by the USMC trying to improve accuracy with hardball in Service Rifle matches versus M14s.
Two issues with "over stabilising"
If you have a 1:7 twist and shoot 40gr rounds there is a known problem where the round can mechanically fail within a short distance of leaving the barrel.
The round is spinning so fast it effectively disintegrates.
The other potential issue with high spin rates in smaller rounds is due to increased precession. This is where the round is not only rotating around it's own axis but also moving around the axis itself. In effect the tip is "waggling" or describing small circles as it moves forward.
If you have ever seen a gyroscope in action you will have seen this.
Spin the gyrosocope up and then stand it up on a mount or rod. As the gyroscope spins the top will start to slowly spin in small circles.
Picture is better than a thousand words....
http://www.nennstiel-ruprecht.de/bullfly/fig15.htm
This is most apparent in rounds longer than they around wide
The two links show a 9mm supersonic round and a .308 supersonic round
The .308 is noticeably yawing more at that moment
http://www.nennstiel-ruprecht.de/bullfly/fig2.htm
http://www.nennstiel-ruprecht.de/bullfly/fig3.htm
In many cases though you will find that a significant factor of groups opening up can be laid at the feet of inconsistent factory loads.
Take a hundred of your rounds and weigh them, you may be supprised how much variance in weight there can be. A difference of a half a grain of powder can vary pressure by 4-5%
If you have a 1:7 twist and shoot 40gr rounds there is a known problem where the round can mechanically fail within a short distance of leaving the barrel.
The round is spinning so fast it effectively disintegrates.
The other potential issue with high spin rates in smaller rounds is due to increased precession. This is where the round is not only rotating around it's own axis but also moving around the axis itself. In effect the tip is "waggling" or describing small circles as it moves forward.
If you have ever seen a gyroscope in action you will have seen this.
Spin the gyrosocope up and then stand it up on a mount or rod. As the gyroscope spins the top will start to slowly spin in small circles.
Picture is better than a thousand words....
http://www.nennstiel-ruprecht.de/bullfly/fig15.htm
This is most apparent in rounds longer than they around wide
The two links show a 9mm supersonic round and a .308 supersonic round
The .308 is noticeably yawing more at that moment
http://www.nennstiel-ruprecht.de/bullfly/fig2.htm
http://www.nennstiel-ruprecht.de/bullfly/fig3.htm
In many cases though you will find that a significant factor of groups opening up can be laid at the feet of inconsistent factory loads.
Take a hundred of your rounds and weigh them, you may be supprised how much variance in weight there can be. A difference of a half a grain of powder can vary pressure by 4-5%
Jim Watson
Member
I have blown up tender match bullets out of a 6.5 twist Long Range barrel, but I doubt you are going to have any trouble with common hardball. After all, the Army had to be able to shoot leftover 55 gr bullets in 7 twist barrels when they went to the M16A2 and later.
The Internet Range is oversensitzed to twist rates which only matter at the great extremes. If you shoot too long a bullet through too slow a twist, it will not be stable. If you shoot too fragile a bullet (not always light weight) from too fast a twist, it can blow up. But there is a wide range of good enough in there, especially if you are interested in shooting on the cheap.
The Internet Range is oversensitzed to twist rates which only matter at the great extremes. If you shoot too long a bullet through too slow a twist, it will not be stable. If you shoot too fragile a bullet (not always light weight) from too fast a twist, it can blow up. But there is a wide range of good enough in there, especially if you are interested in shooting on the cheap.
See the graph I posted. It shows a 55gn and 62grain tested with various twist rates. As noted, 1:9 shoots both 62gn and 55gn equally well. Going to a faster twist will result in more dispersion of the lighter bullet. 1:9 is the best choice if you will mostly be shooting milsurp and equivalent.
If you want to shoot anything over 69gn (depending on ogive, boat tail and composition), you need a faster twist.
If you want to shoot anything over 69gn (depending on ogive, boat tail and composition), you need a faster twist.
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GunTech
Yes, I looked at the graph but the lack of a scale on the radius axis leaves me wondering about the practical effects of "overstabilization." The graph shows a sweet spot but no indication of whether that's 1/2 moa better than worst case or 2 moa.
In the most concrete terms, individual barrel preferences aside, could it be the case that a 1:7 could NOT shoot 1 moa at 100 yds w a 55gr but could with a 68gr bullet?
/B
Yes, I looked at the graph but the lack of a scale on the radius axis leaves me wondering about the practical effects of "overstabilization." The graph shows a sweet spot but no indication of whether that's 1/2 moa better than worst case or 2 moa.
In the most concrete terms, individual barrel preferences aside, could it be the case that a 1:7 could NOT shoot 1 moa at 100 yds w a 55gr but could with a 68gr bullet?
/B
A 1:9 barrel will shoot 55g thru 69g....Your report of a 1:9 BM shooting 55g
"...reasonably well but not great..." probably has more to do with the
ammo or operator than the twist.
My Krieger NM SS 1:9 shoots 55g AND 69 (BH sierra MK HP) just fine.(both sub MOA -consistently)
Now, IF you are gonna dedicate a barrel for heavier bullets,(over 69g)
then go for a 1:8 or 1:7.75....but for the best bang for your buck,
the 1:9 will give you the largest practical spread
good luck
docgary
They won't shoot match accuracy but they're good for practicing.
DMK
Member
everallm, I think you meant to link to these two diagrams:
7.61x51 yawing motion:
http://www.nennstiel-ruprecht.de/bullfly/fig21.htm
9mm yawing motion:
http://www.nennstiel-ruprecht.de/bullfly/fig23.htm
That's a very interesting site. I read it a while back and forgot all about those yaw charts.
However, I'd imagine that those different bullets were shot at different twists. But still, it does describe the inherent instabilities nicely.
I wonder how varying twist rates with that same bullet would affect the yaw factor. If you spin a gyroscope faster, does the tip wobble more or less?
I have to respectfully disagree.
At short range, the effects of spin drift and precession aren't a large factor, but overspin a bullet at 1000 yards and you'll botice the difference. You want the axis of rotation to be tangential to the trajectory as much as possible or drag increases by quite a bit. And while spin drift is present in any bullet, overstabilizing a bullet dramatically exaggerates this.
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