Over-stabilizing a bullet?

[squirrelduster]

I over stabilized a ground squirrel yesterday.

 

[AlexF]

Faster spin bullets = lower accuracy

I just finished a new build. It's a 6mm Ackley Improved with a 1-8 twist. I originaly wanted to shoot 105s and 107s but the throat is a little short so, thinking that the longer bullets would eat up some case capacity, I tried some 75 Vmax and 75 Sierra HPs. The Sierras flew incredibly well. A guy that has a similar set-up was surprised that I could get a lighter bullet like a 75 to fly so well out of such a tight twist. I know a tighter twist robs a little speed but can you over-stabilize a lighter bullet to where it ruins accuracy??

You can't overstabilise a bullet but spinning a bullet faster than minimal safe stable rotation speed degrages accuracy and a bullet can just blow up in flight and disappear!

In short, you can't overstabilise a bullet but

1. Excessive spin = degraded accuracy,
2. Bullets can just blow up and disappear in flight! (not often)

Finally, if you have any interest in long range shooting or hunting you should read Brian Litz's "Applied Ballisctics for long range shooter". He's the ballistician at Berger, a shooting champion and covers everything that you want to know about stability, accuracy, etc.

Alex

 

[LRSickle]

Re: Faster spin bullets = lower accuracy

You can't overstabilise a bullet but spinning a bullet faster than minimal safe stable rotation speed degrages accuracy and a bullet can just blow up in flight and disappear!


In short, you can't overstabilise a bullet but

1. Excessive spin = degraded accuracy,
2. Bullets can just blow up and disappear in flight! (not often)

Finally, if you have any interest in long range shooting or hunting you should read Brian Litz's "Applied Ballisctics for long range shooter". He's the ballistician at Berger, a shooting champion and covers everything that you want to know about stability, accuracy, etc.

Alex

Good comment Alex. I thought about bullet-wringing and slumping but that's pretty rare with lighter bullets. I too am with the understanding that you can't overstabilize a bullet, but I had to throw that question out to see if anyone knew what was going on. I contacted Walt Berger the other day and he recomended a load for the Berger 90gr Target bullets. I loaded some up and Lisa and I are now in Wyoming looking for P-dogs, putting the 90 Bergers to test and verifying some loads for her 6.5x284, .243Win and my 338 Edge. I'll let you guys know how the Bergers do.

 

[Bart B]

Re: Faster spin bullets = lower accuracy

You can't overstabilise a bullet but spinning a bullet faster than minimal safe stable rotation speed degrages accuracy and a bullet can just blow up in flight and disappear!

You can certainly overstablilze a bullet if you spin it so fast it doesn't nose over at the top of its trajectory. Such bullets tend to keep their long axis parallel with the bore axis they were fired from. Lots of drab way down range and accuracy's deplorable.

One other thing, spinning a bullet that's even the tiniest bit unbalanced causes it to wobble/nutate in flight. That adds more surface area going forward and causes more drag. All bullets (save maybe one or two from a box) do this to some small amount. This is why BC's derived from time of flight have a 1 to 2 percent spread, even when muzzle velocity spread is only 15 toi 20 fps. Tests with bullets leaving at the exact same velocity have different times of flight between two points down range; proof their drag or BC aint' all the same.

Best thing to do is not spin bullets any faster than what's needed for best accuracy. Mosts bullets are spun too fast for best accuracy 'cause they're fired from factory barrels that have too fast of twist for most bullets used in them. Took a .300 Win. Mag. out some years ago with a 1:13 twist 26" barrel shooting Sierra 180-gr. HPMK's at 1000 yards. 15 to 20 shots went inside 8 inches; decent enough for new, virgin brass.

 

[AlexF]

Re: Faster spin bullets = lower accuracy

You can certainly overstablilze a bullet if you spin it so fast it doesn't nose over at the top of its trajectory. Such bullets tend to keep their long axis parallel with the bore axis they were fired from. Lots of drab way down range and accuracy's deplorable.

One other thing, spinning a bullet that's even the tiniest bit unbalanced causes it to wobble/nutate in flight. That adds more surface area going forward and causes more drag. All bullets (save maybe one or two from a box) do this to some small amount. This is why BC's derived from time of flight have a 1 to 2 percent spread, even when muzzle velocity spread is only 15 toi 20 fps. Tests with bullets leaving at the exact same velocity have different times of flight between two points down range; proof their drag or BC aint' all the same.

Best thing to do is not spin bullets any faster than what's needed for best accuracy. Mosts bullets are spun too fast for best accuracy 'cause they're fired from factory barrels that have too fast of twist for most bullets used in them. Took a .300 Win. Mag. out some years ago with a 1:13 twist 26" barrel shooting Sierra 180-gr. HPMK's at 1000 yards. 15 to 20 shots went inside 8 inches; decent enough for new, virgin brass.

Good point, it's often mentionned that 55gr FMJ are over-stabilized in 1-7 inch twist and tend to keyhole. I've done some research and found out why!!!

It has to do with bullet quality, for high quality bullet stability factor S of 1.4 to 1.7 is optimal for accuracy and they will stay accurate up to a stability factor S of 3.

It's possible to overstabilize a bullet especially a lower quality one!
Conclusion: shot high quality bullet if you want consistent behavior.

Finally, I'm Canadian and often shoot at -25C (that's -15F) where we get alot of problems stabilizing VLD bullets.

Alex