5.56 Ballistics Question

[HGM22]

Let's say a hypothetical 5.56 round has a "neck" of 1.5'' (that is, the wound track prior to expansion/fragmentation). What happens to this neck as distance to target (let's say a gel block) increases? Does it get larger since the bullet's velocity is slowing down?

Correct me if I'm wrong, but as distance increases/bullet slows the neck increases, expansion and penetration roughly decrease as well (see below) until a stability threshold is met and straight-line "ice pick" wound tracks are seen?

Also, at some point the velocity will be reduced to the point that the 5.56 bullet will not yaw or fragment, correct? Does this then mean the bullet will penetrate even deeper despite having a lower velocity? For example, let's say a bullet at 2800 fps yaws when it hits the block, and penetrates 12''. Would a bullet below the yaw threshold (let's say 2700 fps) actually likely penetrate deeper than 12'' even though its going slower because its not yawing?

 

[Warp]

Let's say a hypothetical 5.56 round has a "neck" of 1.5'' (that is, the wound track prior to expansion/fragmentation). What happens to this neck as distance to target (let's say a gel block) increases? Does it get larger since the bullet's velocity is slowing down?

Correct me if I'm wrong, but as distance increases/bullet slows the neck increases, expansion and penetration roughly decrease as well (see below) until a stability threshold is met and straight-line "ice pick" wound tracks are seen?

Also, at some point the velocity will be reduced to the point that the 5.56 bullet will not yaw or fragment, correct? Does this then mean the bullet will penetrate even deeper despite having a lower velocity? For example, let's say a bullet at 2800 fps yaws when it hits the block, and penetrates 12''. Would a bullet below the yaw threshold (let's say 2700 fps) actually likely penetrate deeper than 12'' even though its going slower because its not yawing?

What bullet?

 

[Tirod]

We are assuming there's just one bullet and can focus on that one bullets performance.

A milspec steel core bullet designed to break apart at the cannelure performs differently than OTM performs differently than one with an expansion point. Lead performs differently than a gilding metal jacket solid with mechanically enhanced petals.

That's where a discussion on the lethality of 5.56 from short barrels gets bogged down - too many assume it's milspec ammo with a high FPS fragmentation threshold, where the actual ammo might be a brass composition hollow point with expansion below 1800 fps. I'm sure not using 62 gr tin/bismuth steel penetrator issue ammo while hunting deer with a 10.5" AR pistol. (By the way - it's not FMJ now. http://bulletin.accurateshooter.com...ues-new-m855a1-ammo-to-troops-in-afghanistan/)

Exposed steel penetrator tips are another wrinkle.

If the bullet isn't specified then it's all speculation about it's performance.

 

[Art Eatman]

If I wuz a guessin' fella, I'd figure that the main change in bullet behavior with longer distance to impact would be less expansion and little change in the "neck".

If it's a breakup bullet or a blowup bullet, I figure the "shrapnel" wouldn't travel as far in the meat.