Bullet Stability?

[coop2564]

Question. Using programs like JBM for stability if your bullet leaves the barrel in the green 1.3 or higher is your bullet likely to hold stable its entire flight until maybe it goes south of SOS. Or if it leaves stable at say 1.33 but at 300yds as it velocity drops if you plugged that velocity in say 2200 the program would then show marginally stable at 1.1. would you expect the bullets grouping ability to decrease past 300?

I guess a better way to ask it is in Long range shooting should your bullet stay in the 1.3 range or higher as its velocity decreases during the flight or is it only important that it be stable when it leaves the muzzle?

Just wondering if I should pick a bullet that would stay above 1.3 its entire flight distance I expect to shoot distance wise for best long range groups?

 

[Rich Coyle]

tag

 

[Mikecr]

Gyroscopic stability goes up with distance, until hitting the next challenge to dynamic stability(usually transonic). The reason is because spin slows less than velocity slows.

Look at gyroscopic stability as displacement per turn (to overcome that displacement). If your barrel is 8tw, that's 8" of displacement per turn, as a bullet is released at the muzzle.
By 300yds, the bullet has slowed, but the turns have not as much, and the effective twist rate could be 6tw by then(only 6" of displacement per turn).
The biggest challenge to stability, besides hitting your mark, is muzzle release. This is why bullets that are not stable tumble immediately (never making it to stable). If you make it past a chronograph without tumbling, chances are the bullet will continue point forward, truing out as it goes onward. Some refer to this as 'going to sleep'.

 

[BallisticsGuy]

^^^ that. spin rate (ignoring distance covered, so spin, not twist I gues) doesn't drop much at all. Not much friction there. If you consider distance covered with bullet rotation then like the man said, things actually tighten up on the long end even though the rotational rate changes only slightly. Like a coarse thread versus a fine thread being cut on a lathe. Pitch is determined by the speed of the rotation of the shank (the thing being threaded) and the speed that the cutter is pulled down the length of the shank.

FWIW, 1.3Sg is what I'd consider marginally stable. In my experience I get more predictable transonic zone performance with calculated Sg's around 1.3-1.4 but you lose BC by a tiny bit. Getting around 1.5 seems according to Berger to be where you're getting your full advertised ballistic coefficeint. I'm assuming this is due to the minimized yaw precession that comes with full gyroscopic stability.

 

[coop2564]

Thanks. I think I got a good handle on it now.

 

[Bullet bumper]

Another big challenge to bullet stability in any particular twist rate is internal bullet construction . Some bullets are just not made right and some are made in a way that requires extra twist rates that they should not require for their length .
You can calculate based on velocity , twist rate and bullet length and generally it is spot on but if the bullet is made inside in some unusual way then that calculation could be flawed .
As a matter of interest sometimes bullets like boat tails do not fly point forward initially they fly point slightly up initially .
How far they fly like that before they flatten out I don't know .
However it must affect actual in flight BC to some small degree .