AR15A2 vs M16A2(&up) / M4 specs ?

N

nighthalk

Inactive
Fellow shooting lovers:

Don't mean to beat this issue to death, but:

223 vs 5.56 specs are still a mystery to me. It seems the inside 5.56 case
is thicker elimating a tiny amount of powder volume and that's all. I think there is more difference dimesionally or something?
pls help me with the "Eternal mystery of the sea.

Let's assume: Colt civilian/enforcement only, highest quantity made, largest scale production run barrels only, and focus on models below only:

Colt AR15A2 - military weight barrel (no variants), 1 per 7in. NATO twist, chrome chamber/barrel.

Colt M16A2 (fwd)/M4 same as AR15A2 above features.

Tks in advance rich---------end-rtlsr---2/8/06---------|
 
OK, I'll try to explain it. The external case dimensions are the same for .223 Remington and 5.56x45mm (AR 15/M16 round). Both fire a projectile of .224 inches. Here is the difference...the chamber of the .223 and the 5.56 are basicly identical, but the rifle bore is where the diff is. The barrel is initially bored out to .223" and then rifled with grooves that measure .224". The 223 lands & groves start a very short distance from where the projectile enters the chamber/bore with the bolt locked. The 5.56 lands (.223" diameter) start farther down the bore, sort of like the free-bore on a Weatherby magnum rifle. This leade (yes, it is spelled correctly), or free-bore, allows the projectile to exit the case and jump forward a short distance before being forced into the rifling. This allows more pressure to be generated, thus more velocity. In addition, the case walls of the 5.56 round are a little thicker to hold a bit more pressure. You can fire either round from a rifle chambered for 5.56, but firing 5.56 rounds in a rifle chambered for .223 will result in somewhat dangerous pressures, and could damage the rifle and possibly injure the shooter if the weapon explodes in his/her face. Hope that helps!
 
The military round uses its bore diameter in its designation; 5.56 mm. That's not .223". Do your own math. That's what the military barrel's bore diameter is before it's rifled when broach, button or single-point cut riflers are used. Hammer forged barrels end up with that bore diameter. Their groove diameter's the same as the commercial version. And there's about .001" tolerance, too, for both. Same thing for the 7.62mm NATO rifle round.

SAAMI specs for the .223 Rem's bore diameter is .219" and the groove's .224" Both have a +.002" tolerance.

Regarding bullet jump to the rifling. The more there is, the more the bullet deforms when it slams into the rifling. Best accuracy typically happens with zero jump to the rifling. But commercial and military ammo has to be totally functional in all sorts of rifles. Accuracy is not at the top of their objective's priority list.

Peak pressure. . . . What's readers thoughts on both rounds for their peak pressure? Use either system; the old one or the new one.
 
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To add to Bart B.'s reply,

The most accurate milspec 5.56 ammo, Mk262 Mod1 uses the 77gr SMK (cannelured) and groups between 1.0 and 1.5 MOA during lot acceptance trials. This is a 50 shot group extreme spread.

Using 5.56 ammo in a 223 chambered AR increases your risk of an overpressure event which will pop a primer that might gum up your rifle.

Jimro
 
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