Stability Question.

[waterfwlr]

I've heard two theories.

One being, if a bullet is stable, it will stay stable.

Second is, a bullet can be stable, and then become unstable at long distance.

Which is correct?

 

[CatShooter]

I've heard two theories.

One being, if a bullet is stable, it will stay stable.

Second is, a bullet can be stable, and then become unstable at long distance.

Which is correct?

Both...

It depends on a ton of variables.

 

[vtmarmot]

Actually, it can become more stable until it becomes less so at a point. If you want the whole story, get Bryan Litz's book "Applied Ballistics for Long Range Shooting." It is a well-written and understandable guide for shooters who want to know more.

 

[waterfwlr]

Thanks. The reason I ask is, I have a marginal twist bullet combo that shoots very well at 100 yards, but wonder if it will stay stable at say 800 yards. If I had a place too shoot that far around here I would just find out for myself.

What are a few of the variables?

 

[broncman]

Ballistics is a very mature science. Bryan litz has a great online calculator that is very good. Bullet length, form factor, weight, BC and twist rate, muzzle velocity are all fixed variables. Air density, temp, humidity and wind are your changing variables.

What bullet, twist and velocity are you running?

 

[waterfwlr]

12 twist, 87gr vmax, approximately 3150 fps.

 

[M-61]

Thanks. The reason I ask is, I have a marginal twist bullet combo that shoots very well at 100 yards, but wonder if it will stay stable at say 800 yards. If I had a place too shoot that far around here I would just find out for myself.

What are a few of the variables?

If Calverton is still open you can see what happens at 200yds.

 

[waterfwlr]

Thanks. The reason I ask is, I have a marginal twist bullet combo that shoots very well at 100 yards, but wonder if it will stay stable at say 800 yards. If I had a place too shoot that far around here I would just find out for myself.

What are a few of the variables?

If Calverton is still open you can see what happens at 200yds.

I'm close enough to hear the shots from that range. Not only is that range horrible, I shot at 200 at a different range and was still shooting great.

 

[M-61]

My guess it's the worst range anywhere.......but it is first and foremost a 'place to shoot'. Aside from the lack of any organization, years ago you had F-14's landing and taking off all the time so if there was anyone running the range you could not hear them. (The range officer I mean,those two afterburners were quite clear.)

 

[broncman]

According to bergers online calculator, that combo is on the very bottom edge of marginal stability given a 2000 feet elevation and 60 degree temps.

Marginal stability is in the range of 1 to 1.4. You are barely 1. It will destabilize at long range.....exactly when, no idea???

 

[NateHaler]

Thanks. The reason I ask is, I have a marginal twist bullet combo that shoots very well at 100 yards, but wonder if it will stay stable at say 800 yards. If I had a place too shoot that far around here I would just find out for myself.

Picture this: A very accurate rifle chambered in .308 Win, featuring a 1:12" bore twist, and Fed Gold Medal Match 168 ammo. Capable of match-winning accuracy to 600y. (In fact, I was leading such a match, including being ahead of the guy who ultimately won the match with a range record, to 600y. Shooting factory loads because I didn't have time to make enough 175 handloads per usual...)

At 800y-1,000y, my groups went from sub-MOA to measure with a yardstick, due presumably to transonic issues. Whether that's "instability" I don't know, but whatever it was killed my chances to win.

 

[M-61]

Thanks. The reason I ask is, I have a marginal twist bullet combo that shoots very well at 100 yards, but wonder if it will stay stable at say 800 yards. If I had a place too shoot that far around here I would just find out for myself.

Picture this: A very accurate rifle chambered in .308 Win, featuring a 1:12" bore twist, and Fed Gold Medal Match 168 ammo. Capable of match-winning accuracy to 600y. (In fact, I was leading such a match, including being ahead of the guy who ultimately won the match with a range record, to 600y. Shooting factory loads because I didn't have time to make enough 175 handloads per usual...)

At 800y-1,000y, my groups went from sub-MOA to measure with a yardstick, due presumably to transonic issues. Whether that's "instability" I don't know, but whatever it was killed my chances to win.

I am not a 1000yd shooter. However I thought the ability for the bullet to remain super sonic at 1000yds was an absolute necessity. Should it not the transition to sub-sonic allows the sound wave to 'catch' up to the bullet and when it does it upsets the bullet, and it's game over for the accuracy.

 

[waterfwlr]

I'm talking about supersonic velocity. I don't want to complicate this question even more.

 

[CatShooter]

The trans sonic "theory" does not meet the test of physics.

Sound waves cannot catch up to the bullet and tumble it. Nor is there any "turbulence" for the bullet to go through... leave that to old B&W WW-II movies.

As the bullet slows down, the the sound/shock waves from the nose and tail get weaker and weaker, and when the bullet is close the the speed of sound, the shock waves are virtually non-existent... and they cannot "Catch-up" to the bullet because they were always going away at 90° to the bullet's direction.

The transition from supersonic to subsonic is smooth and soft.

The 168 SMK is not a particularly well designed bullet - folks have been complaining about for 30+ years.

If you shoot it in a 10" twist, it is fine past 1,000 yards.

 

[NateHaler]

The 168 SMK is not a particularly well designed bullet - folks have been complaining about for 30+ years.

The FGMM 168 SMK, i.e. .30 caliber, is well-enough designed for excellent performance to 600y. Past that, the 175 SMK is clearly superior, at least in a .308 at safe pressures.

 

[CatShooter]

The 168 SMK is not a particularly well designed bullet - folks have been complaining about for 30+ years.

The FGMM 168 SMK, i.e. .30 caliber, is well-enough designed for excellent performance to 600y. Past that, the 175 SMK is clearly superior, at least in a .308 at safe pressures.

With a 10" twist, the 168 will fly to the horizon and be accurate. The 12" twist was a military decision, and is OK for hunters, but it is not the best for civilian match shooters.