Theoretical ballistics question

[MacArther]

A crazy sort of thought crossed my head after hearing about the .30 SABOT like device in the .50 SLAP round (correct me if I got the round ID wrong): What if someone designed a rifle around the 5.56mm NATO bullet, but with the charge (/casing, whatever the correct term is) of a .50 cal? I mean, neck the cartrige so it fits the bullet, not turn the bullet into a submunition for penetrating armor. What sort of tollerances would the weapon need theoretically, and what sort of performance might be gained?

[renrich]

You would get a lot of velocity out of a .50 BMG case necked down to a 80 gr (or something) 5.56 bullet, probably more than you could use. Around 4000 ft/sec is getting close to the practical limit of velocity. You start having issues like bullet instability, barrel erosion and others. The energy generated by that size bullet would not be all that great in spite of the high velocity because of the light weight. There are a lot of wildcats(that is what a cartridge like that is called) but not many that extreme. If I remember right there used to be a round called the accelerator that was a 30-06 case with a .22 bullet that generated 400 ft/sec.

[renrich]

Left off a zero. 4000 ft/sec

[MacArther]

So, assuming there were strong enough materials to make both barrel and bullet out of, would it be effective for military applications? Or would going to a bullet size of 6.8mm or 6.5mm be better for instability?

[renrich]

Part of the problem with that idea is that the action of a rifle made for that case and load would be quite bulky and the recoil would be fierce. More than the average man could stand unless the weapon was so large and heavy it would not be suitable for military use. A bullet with that velocity would not have much penetration as it would likely disentegrate upon impact.

[delcyros]

... as Renrich pointed out, the projectile, moving with a speed close to the point when the filler explodes (except a solid shot) is not a good solution at all.
You could calculate barrel length, riffling, and best weight for a specific muzzle velocity but there is little sense for such a tool.
It could be useful as an easily portable, long range sniper weapon but here again, the low weight of the projectile would cause larger errors in the mean point of impact, all other factors beeing identic.

[renrich]

The US is already using sniper rifles In Iraq chambered in .50 BMG. The ballistic coefficent of that bullet is so high that shots at 1000yds or more are possible. As Del says, a .556 bullet would be much more susceptible to exterior conditions.

[Jank]

In addition to other points raised above, you would need a long, long, long barrel to take full advantage of the acceleration of the bullet in front of that much gas through such a tiny bore. Throat and barrel erosion would be pretty bad too.

I frankly do not see a practical application. Now a .50 BMG necked down to .338 with a 300gr. projectile would be interesting!

[MacArther]

What about re-using the German early anti-tank rifle concept of a 7.92mm bullet attached to a 13mm case? Would this be a viable long range sniper round?