bullet stabilization - where/when do you (not) see it

[davsco]

'everyone knows' you need a faster twist rate for the heavier-for-caliber bullets to stabilize them. if you're not using a fast enough twist, at what distances (or speeds, eg going subsonic) does that manifest itself?

i just put together a 224 Valk, 24" 1-9.5 twist. easily sub-moa at 100 and just a hair looser but still easily sub moa at 200, with fed 90 gold medal, 5 shot groups. mv of roughly 2,650fps. because this isn't a 9, or 8.7 or even 8.5 twist, is accuracy going to go to crap as i start reaching out more, or if it works at 100 and 200 it will work at 800 (est mv still well over speed of sound)?

 

[Deleted member 113831]

'everyone knows' you need a faster twist rate for the heavier-for-caliber bullets to stabilize them. if you're not using a fast enough twist, at what distances (or speeds, eg going subsonic) does that manifest itself?

i just put together a 224 Valk, 24" 1-9.5 twist. easily sub-moa at 100 and just a hair looser but still easily sub moa at 200, with fed 90 gold medal, 5 shot groups. mv of roughly 2,650fps. because this isn't a 9, or 8.7 or even 8.5 twist, is accuracy going to go to crap as i start reaching out more, or if it works at 100 and 200 it will work at 800 (est mv still well over speed of sound)?

The Berger stability calculator on thier website will give you some idea. Input your twist and bullet, then see at what speed it becomes marginally stable, then translate that speed to whatever distance in coincides with in your ballistic calculator.

 

[Diver160651]

'everyone knows' you need a faster twist rate for the heavier-for-caliber bullets to stabilize them. if you're not using a fast enough twist, at what distances (or speeds, eg going subsonic) does that manifest itself?

i just put together a 224 Valk, 24" 1-9.5 twist. easily sub-moa at 100 and just a hair looser but still easily sub moa at 200, with fed 90 gold medal, 5 shot groups. mv of roughly 2,650fps. because this isn't a 9, or 8.7 or even 8.5 twist, is accuracy going to go to crap as i start reaching out more, or if it works at 100 and 200 it will work at 800 (est mv still well over speed of sound)?

Just a note, heaver bullets do NOT always require faster twists, longer ones do. You'll notice this with solids.

Unfortunately, with marginally stable lengths, it isn't as simple as plugging number and and your GTG.

While the calculators give you some numbers and staying inside 1.2M helps; you have to shoot your rounds to see what the wind does if you have a bullet on the very edge. in your case you're really asking for a lot, theoretically it is unstable from the start.. You have to just test it, but I would also try it on a windy day, the extra attack angle of the bullets and buffeting might provide a surprisingly different result.

 

[sandwarrior]

'everyone knows' you need a faster twist rate for the heavier-for-caliber bullets to stabilize them. if you're not using a fast enough twist, at what distances (or speeds, eg going subsonic) does that manifest itself?

i just put together a 224 Valk, 24" 1-9.5 twist. easily sub-moa at 100 and just a hair looser but still easily sub moa at 200, with fed 90 gold medal, 5 shot groups. mv of roughly 2,650fps. because this isn't a 9, or 8.7 or even 8.5 twist, is accuracy going to go to crap as i start reaching out more, or if it works at 100 and 200 it will work at 800 (est mv still well over speed of sound)?

Depending on which bullet, you will see it happen from the point it leaves the barrel all the way out to transonic. For instance, I had a 7mm-08 barrel with a 1-11" twist. Just to use up some 162's, I tried shooting them. They key-holed and made 8-9" groups @ 100 yds. At 300 yds. they wouldn't hit an 8 ft. backstop. It was literally like a knuckleball coming out of the barrel. The same barrel would put 154's inside 1.5" consistently.
In my experience, if a bullet will barely stabilize, it generally won't go unstable until the transonic. Which is when the bullet speed drops to around the speed of sound. It starts to be affected above the speed of sound and is affected to slightly below the speed of sound. The reason for this is because of the bullet's shape, air is moving at different speeds around the bullet. Some of it supersonic, some of it subsonic. The shockwaves collapsing on the bullet causes the most disruption.

FWIW, all bullets are affected by decreasing speed in the transonic realm. But, due to rifling twist and airflow design around the bullet, some bullets withstand the disruption a lot better. Some almost to the point of not seeming affected. While others, never restabilize. An example of this was one day shooting an AR15A1 with a 1-12" twist barrel. with 55 gr. fmjBT's The bullets were holding 2-4moa at 700 yds. At 750 yds. the group spread to 10 moa. At 800 the group spread to about 8 ft. (all over the hill around the target) It was a cold day at 2500 ASL. But a perfect example of the range a bullet was no longer effective. The bullet hits were not showing keyholing at 700 but were beginning to show it at 750.

p.s. I found one of the bullets from the 7mm-08 story above. I had shot them in winter and it hit a snowbank about 20 yds. in front of the bunker (backstop). it was in perfect condition except for the rifling marks. Other bullets had hit the berm behind the bunker, both over and alongside.

 

[ncr92]

In his paper on stability factors, Don Miller stated that stability factor is lowest at the muzzle and increases as the bullet travels down range. This is because rate of spin decays more slowly than velocity.

See the second last paragraph on page 5 of his paper:
http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_1.pdf

 

[Lowlight]

Would not bother with a 1-9.5 Twist for a Valkyrie, seems like a waste of time when the guys studying and working heavily with both manufacturing the rifles and the guys with the bullets are doing 1-7

I have about 350 rounds out of my Valkyrie, both here in CO and again last week in AK, so very different conditions. Accuracy is outstanding but they start to see serious wind and open up around 700-800 yards. In CO I shot it to 1125yards and while I had a 50% hit rate you cannot tell you are hitting anything. My 1125 yard target rings like nobodies business, the neighbors can hear it, yet, nothing out of the Valkyrie.

Inside 800 its a hammer, after the wind eats it's lunch until the better bullets are out or loaded. All reports say the 88gr Hornady is the money maker for this rifle.

I have seen guys building 1-6.5 / 6 twist barrels for the Valkyrie to shoot the 90s, but nothing is working for them, at least not as good as they should be. 3/4 MOA is standard out of them but a 50 cent 75gr American Eagle will stack them sub 1/2 MOA without trying. Even the 62gr Varmint Federal Ammo is more accurate currently.

 

[sandwarrior]

In his paper on stability factors, Don Miller stated that stability factor is lowest at the muzzle and increases as the bullet travels down range. This is because rate of spin decays more slowly than velocity.

See the second last paragraph on page 5 of his paper:
http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_1.pdf

That is true...out to the point where the bullet goes transonic.