The UK's Finest Independent Forum for Shooters and Gun Owners. Est 2010.

What factors govern the stability of a bullet in flight? We're assuming the rotation rate and mass have the greater effects, but how so, and are there others?

Might it be possible to have a barrel with a correctly rate of twist which could stabilise adequately a supersonic bullet and a slower, longer, heavier bullet too ? Our thinking is that we might like rifle that can shoot 6.5x55 Swedish milsurp ammo in one guise, but which could fire a sub-sonic round too - assuming that sunsonic round was longer and heavier...

Badger,

I guess you have already tried the available charts like the Shillen one?

Available charts?

*looks intrigued, scurries away to google*

Hmmm, we need to ponder a little more. We're hoping we can get reasonably good accuracy from a barrel by using supersonic ammo of one bullet weight / profile and length, and then changing to subsonic ammo of a different bullet weight / length / profile. Are we deluded?

Badger
Overall bullet length is a major element
Imagine the ease with which you stabilise a spinning top versus the speed to spin a pen
If you look at the Berger 155.5 versus 155 hybrid
Recommended twist differs 155.5 is 13 and 155 hybrid is 1:12
The difference in is only bullet length 1.25 versus 1.280

I've heard that bullets can de-stabilise as they are slowing down through the speed of sound, I'm guessing this happens at long ranges for the large calibres tho ..

zzr1100 wrote:I've heard that bullets can de-stabilise as they are slowing down through the speed of sound, I'm guessing this happens at long ranges for the large calibres tho ..

Yes. But you have to stabilise them first for supersonic flight

Lack of stabilisation will mean they fly less than optimally, bleeding speed and getting to the transonic barrier sooner than is optimal

Quite correct, oomans, quite correct.

Our intention is that the firing of sub- and super-sonic rounds will be on different occasions, and with bullets / loads optimised if possible for those events. The same rifle and barrel will be used for both.

Sub sonic lead bullets are heavier and shorter
Any barrel that is fast enough to stabilise a longer copper lead combo will likely be for adequate

Any bullet flying below speed of sound has a different frontal bo wave