Thread: Stability for subsonic projectiles

Gents, can someone pls confirm the following:

Is stability factor the thing that causes subsonic projectiles to keyhole or tumble ? I guess the question is if the stability factor applies in the same way at subsonic speeds as for high velocity ?

I've been playing with the JBM stability calc (keeping velocity constant at 1050fps), and it seems that:

for a given projectile length, the heavier the projectile, the more stable it is
for a given projectile weight, the longer it is the less stable it is

If you use a projectile that according to the calc should be ok, what other factors can cause it to tumble ?

Center of gravity, If its marginally stabilized and has a massive hollow point then the bum is heaviest and on impact it will turn and penetrate backwards, this can happen in the first 2 or 3 inches of penetration then they just sail on threw backwards rather than actually "tumbling" they just turn around.

Big stacks of wet newspaper are fun and are great for seeing what actually happens.

Wound channel created by this was much more impressive than any that turned.

Yes , you need a faster/tighter twist to stabilise the bullets , I think its to do with the rpm of the bullet , remember a say 175gr 30cal bullet at say mach 2.5 , is being spun at 2.5 times more than the sane bullet at just under mach 1 .

Its to do with not just the projectile weight but also to a lesser degree the bullet/shape/ , ie bearing surface exposed to the rifling directly , thinl say a VLD jacketed compared to say a flat based cast RN bullet , the cast will have 2-3 times the surface in contact to the rifling .

If you look at say the 30 cal , at say 2700fps , 175gr SMK , a 1-12 twist is fine , and if say you wanted to run subsonic ( and to compensate for reduced speed , you normally go as heavy as you can ) , then you would need say 1-7 twist for say a 240gr bullet at 1000fps .

If you have a bullet of the same weight , and ones a VLD shape & the others a less streamlined cast RN flat base shape , then the cast bullet with more surface contact with the rifling will be easier to stablilise in a marginal twist rate .

The other factor that can cause timbling is if the bullets contact the baffles inside your suppressor , due to either mis-alignment issues or in-correct twist rate to fully stablilise the bullet .

As you will see , its ticky .


Later Chris

I can get away with 1:10 for 240gr SMK's out of my 30-223 subsonic, but all things being equal it shouldn't actually stabilise as it's meant to be min 1:8 twist...but shhhh, don't tell my rifle that or it may start tumbling them eh?

kj

I read somewhere (can't rem) that with the AK74's 5.45 and the 300 Whisper, bullets tumbling through the target make them quite effective.

Originally Posted by Chupacabra

I read somewhere (can't rem) that with the AK74's 5.45 and the 300 Whisper, bullets tumbling through the target make them quite effective.

Compared to just penciling threw maybe.
In my testing they turn around in the First couple of inches then just sail on backwards doing no real damage.
Tumbling has got nothing on a bullet that will expand and fragment.

My research and Cams recent testing of my bullets have shown a 1:11 will happily stabilise a 22xgr projectile
Bearing surface has little to do with it .... It's all about the balance point. A VLD profile with a light bum will stabilise in about anything
But in regards to terminal performance, the balance point is what makes or breaks as far as turning or pushing through straight and expanding

Some bullet designs are more stable at subsonic than supersonic velocities, some are more or less the same and some are very unstable at subsonic. The nose shape has a lot to do with it i.e. round nose, spire points and flat nose (hollow noses are like flat noses). Some bullets increase stability with increased velocity and some decrease stability as velocity increases. I forget which nose shapes do what and the online calculator that showed the effect has gone off-line.