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Old September 21, 2020, 11:48 AM   #1
ghbucky
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over-stabilizing bullets?

I see bullet manufacturers talk about x bullet is recommended only for barrels with an x twist or faster. I've never seen one say that x bullet shouldn't be shot faster than a certain twist rate.

Yet I know that there is apparently a 'too fast' effect. But I don't know what is actually happening.

Can someone enlighten me why a 55gr fmj bullet out of my 1:8 16" barrel is 6+ MOA while a 63gr is 1 MOA?

1 of the 55gr holes was kind of oval, so I guess the bullet was yawing?

What is going on during flight?
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Old September 21, 2020, 01:35 PM   #2
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https://www.ar15.com/ammo/project/perf_twists.html

https://www.ar15.com/ammo/project/perf_m193twists.html

https://www.ar15.com/ammo/project/perf_m193acc.html

https://www.ar15.com/ammo/project/perf_whattwist.html

From that last link, this is the statement I was thinking of when I responded to your comment in another thread:

Quote:
1:12 twist rifles cannot stabilize SS-109/M855 bullets and 1:7 twist rifles are slightly less accurate with lighter bullets and will often blow apart the thin jackets of lightweight varmint bullets. The 1:7 twist is used by the military to stabilize the super-long L-110/M856 tracer bullet out to 800 yards, but unless your plans include shooting a significant amount of M856, the 1:9 twist rate is better suited for general use.
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Old September 21, 2020, 01:46 PM   #3
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When fired at 3200 fps in a 1-in-7 twist rifle, a round is rotating at over 300,000 rpm when it leaves the muzzle
wowzers!
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Old September 21, 2020, 02:28 PM   #4
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Lets just say it's harder to spin a bullet too fast than too slow. It is usually better to err on the side of a twist a little faster than optimal.

But a 1:8 twist should shoot pretty much any 50-75 gr bullet just fine. I know my rifles do. You might see issues with bullets on the extreme ends of the weight spectrum. Say under 50 gr and some of the 90 gr bullets.

If you're having issues with some 55 gr bullets the problem isn't the twist. Just inaccurate bullets.
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Old September 21, 2020, 02:53 PM   #5
ghbucky
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If you're having issues with some 55 gr bullets the problem isn't the twist. Just inaccurate bullets.
Its Federal American Eagle 55gr FMJ. Not premium, but I would have thought it would do better than 6 MOA. And one was definitely yawing. One of the links AB provided shows that an over spun bullet will stay nose up throughout flight, and that is what the profile of the paper seemed to show.

Also, Berger bullets has a stabilization calculator that says a 55gr Sierra fmj will only be marginally stable out of a 1:8 barrel.

https://bergerbullets.com/twist-rate-calculator/
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Old September 21, 2020, 05:17 PM   #6
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When fired at 3200 fps in a 1-in-7 twist rifle, a round is rotating at over 300,000 rpm when it leaves the muzzle
That's almost as bad as 18 million revolutions per hour!
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Old September 23, 2020, 08:17 AM   #7
Bart B.
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3200 X 720 / 7 = 329,142.8 rpm.

A 1:14 twist is about perfect for that ammo. Your rifle spins them so fast they jump off the bore axis from excessive centrifugal forces.

The 222 Remington shot 50 and 55 grain bullets a little slower in benchrest match barrels with 1:14 twist. Won benchrest matches.

Your barrel is twisted for the heaviest bullets
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Old September 23, 2020, 08:26 AM   #8
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Do you have a ballistics chart or program that USES bullet RPM?
All I get out if it is gee whiz big numbers, everything is specified in twist and or velocity.
(I went round and round on that the other day.)
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Old September 23, 2020, 09:59 AM   #9
Bart B.
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Originally Posted by Jim Watson View Post
Do you have a ballistics chart or program that USES bullet RPM?
All I get out if it is gee whiz big numbers, everything is specified in twist and or velocity.
(I went round and round on that the other day.)
No. But this one calculates rpm to use in its calculation:
https://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

Whatever combination of bullet, twist and velocity calculates a stability factor of 1.35 to 1.45 is best for accuracy.
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Old September 23, 2020, 11:51 AM   #10
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Quote:
Originally Posted by Bart B.
No. But this one calculates rpm to use in its calculation:
https://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

Whatever combination of bullet, twist and velocity calculates a stability factor of 1.35 to 1.45 is best for accuracy.
Does that work for flat-base handgun bullets, too, or is it only valid/useful for boattail rifle bullets?
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Old September 23, 2020, 12:18 PM   #11
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Its Federal American Eagle 55gr FMJ. Not premium, but I would have thought it would do better than 6 MOA.
May not really apply, but consider this,
I have a 1970s Army manual that states (one of) the Standards for Overseas Shipment of the M16A1 rifle was 8 MOA (eight moa).

If the rifle wouldn't shoot at least that well, it was not deployed overseas, but retained in CONUS for training use.
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Old September 23, 2020, 12:24 PM   #12
Bart B.
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Quote:
Originally Posted by Aguila Blanca View Post
Does that work for flat-base handgun bullets, too, or is it only valid/useful for boattail rifle bullets?
Yes, as far as i know, it works for all bullets.

Frankfort Arsenal knew all the 30-06 military bullets from 24 inch 1:10 barrels were spun too fast so the 7.62 NATO ammo shot virtually equal bullets about 100 fps slower with a 1:12 twist barrel 2 inches shorter then got better accuracy.

Last edited by Bart B.; September 23, 2020 at 12:29 PM.
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Old September 23, 2020, 12:30 PM   #13
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Quote:
I have a 1970s Army manual that states (one of) the Standards for Overseas Shipment of the M16A1 rifle was 8 MOA (eight moa).
Wow, OK so I guess that ammo is performing as expected.
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Old September 23, 2020, 01:27 PM   #14
Jim Watson
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I plugged in the values for .30 Luger and got a stability factor of 6.
A lot of Service Pistol and USPSA Open guns are getting 32 twist barrels for a 9mm stability factor in the 1.5 range like a rifle.

Back in the days of the 1903 Springfield, they machine rest tested .30 rifles, spot check on service rifles, all Sporter, NM, and Free rifles.
There is a legend that one month they noticed that accuracy was better and they wondered why. Checking back, they found that the machinist setting up the rifling machines had used the wrong sine bar and turned out a number of barrels with 11 twist. So what did they do? Revise the spec? No, they readjusted the rifling machines for the standard 10 twist.

Last edited by Jim Watson; September 23, 2020 at 01:39 PM.
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Old September 23, 2020, 09:02 PM   #15
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Quote:
I have a 1970s Army manual that states (one of) the Standards for Overseas Shipment of the M16A1 rifle was 8 MOA (eight moa).
The current standard is 4moa. That being said, I’ve touched a ton of rifles stamped US Property, from M16A2s, to FN manufactured M16A4, to Colt M4s (whatever version we’re on now, all the same over the past 20 years imo). Never found one that shot worse than about 2moa.
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Old September 23, 2020, 09:59 PM   #16
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Wow, OK so I guess that ammo is performing as expected.
More like it wasn't performing any worse than allowed by that particular Army standard.

Your 55gr ammo might group better in a rifle designed to shoot the 55gr bullet. Each bullet weight (and length) has an optimal twist rate, and will work, to a degree in a range of twist rates. For some bullets that range is wider than others.

Yes, it is possible to spin a specific bullet "too fast". Even a "solid" bullet is still a pointed cylinder being spin around its notional long axis. And nothing made by man is 100% perfect, so the faster it us spun the greater the effect of any imperfections on the "wobble".

Some years back I got several boxes of the Sierra 63gr "semi spitzer", They were the "deer bullet" in .22 cal, at the time. Shot some from muy .22-250. Regular Win M70 varmint, with the twist for the 50-55gr slugs. That rifle, 3/4" with 55gr sp and less with the match 52/53gr HP bullets.

The long semi spitzers shot into 2 inches never really any better. Minute of Deer so all things considered, I wasn't unhappy.

Now everyone says this was because of the twist in my rifle being "too slow" for that bullet, and they're probably right. So, if there is a "too slow" then there must also be a "too fast".

Traditional "cup and core" bullets will come apart from too high a rotational stress. How much is too much depends on the construction of the bullet.

Quote:
The current standard is 4moa. That being said, I’ve touched a ton of rifles stamped US Property, from M16A2s, to FN manufactured M16A4, to Colt M4s (whatever version we’re on now, all the same over the past 20 years imo). Never found one that shot worse than about 2moa.
I don't doubt that a bit. The construction of the A2 and variants generally are more accurate than the original M16 and A1.

I never saw, or heard of any of the rifles back then being tested before deployment, and I saw a lot of them go, and saw them overseas. We inspected them, but no one ever shot them to see if they "official standard" was met.

I was a Small Arms Repairman (45B20) from 75-78, Never once had a weapon brought to the shop for any accuracy issue. Function was the rule, and no live fire testing was ever done anywhere I served or ever heard of.

I'm talking about line units and their support, not specialty work like the Marksmanship shop making match rifles.

I think it quite likely that not only were rifles not tested before overseas shipment, but that none of them would have failed, had they been. There are things where the Army has a standard, just to have a standard...
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Old September 24, 2020, 01:47 PM   #17
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"...the bullet was yawing?..." Yep. It's about the weight to diameter ratio. A heavier bullet is longer and needs the faster twist to stabilize.
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Old September 24, 2020, 02:30 PM   #18
Jim Watson
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In a different part of the envelope, my .223 6.5 twist "90 grain gun" will blow up a 75 grain bullet at maximum, but loaded down a bit, they are at least as accurate as the 90s.
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Old September 24, 2020, 04:43 PM   #19
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but loaded down a bit, they are at least as accurate as the 90s
Interesting. I've got a couple of boxes of Sierra 55gr, might be fun to see if I can get them dialed in. I thought they weren't going to be worth messing with.
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