Twist Question

[Optimistic Paranoid]

Any amateur ballisticians here?

My understanding is that the twist of the original M-16 that was first deployed to 'nam would just barely stabilize the 55 grain bullet then in use, and it usually would tumble when it hit the enemy, causing massively more effective wounds.

I've noticed that Hornady, as part of their TAP series of Personal Defense Ammo, is offering a 110 grain 308 round, and I'd like to figure out how to 'run the numbers' to determine just how stable IT would be when fired from, say, a Springfield Armory SOCOM.

I'm thinking that a 30 caliber bullet that tumbles on impact would make for an AWESOME CQB stopper!

Regards
John

 

[TonyD]

The first '16's were a 1:12 (if I recall) and stablized the 55 gr. just fine. Heavier bullets need a tighter twist.

The "tumble" is often misunderstood. The base of the bullet is heavier than the nose is any caliber / weight of bullet of this design. Therefore, the ass-end is always trying pass the nose. Velocity and spin keep this from happening until it becomes very slow and unstable, or encounters obstructions like twigs, brush or bones.

Contrary to some belief, the .223 is not a good home defense round because it does tend to tumble going through walls and change directions drastically. And, yes, it does over penetrate. There are many examples at my range where .223 AND .308 rounds have hit a 4x4 post at 500 yards dead in the center and exit the entire post sideways after traveling that 500 yards and losing all that energy. Just think how much more energy it has at 10 feet.

 

[Nickle]

I pretty much concur with Tony. The tumbling effect takes place upon impact with the M80, M193 and M855 projectiles.

And the 110 grain .308 would be overstablised. The only thing you lose is some accuracy.

Instability is caused by a too heavy bullet for the twist.

 

[M14Robert]

Eugene Stoner's first AR was 1/14" I believe, but the M16s came out with 1/12". As far as a tumbling .30, the 7.62 x 39 is king, with yaw starting out with as little as 2" penetration with the right ammo.

 

[blkbd]

Robert is right the 1st M16's on trial basis were 1 in 14 and they would barely stabalize the bullit and created some nasty wounds, but when testing was done in cold climates the group size opened way up do to the denser air. So the twist was increased to 1 in 12.