AR SBR Terminal Velocity/Energy: .223 vs. 9mm

[techres]

So in the future I plan to SBR a spare lower and it has me looking again at the eternal debate between .223 and 9mm when it comes to a 10-11" AR SBR. Given the usual candidates (9mm is quieter, cheaper per round and lets me do "pistol only" ranges, .223 keeps me from buying more expensive mags and is a caliber that is already in the house), one thing that may help me make up my mind is the terminal characteristics of the rounds in a home defense situation.

So I am here looking for info that people could direct me to on terminal stuff on the two rounds given a 10-11" or so barrel - does not need to be exact comparisons. I am interested in terminal energy (and can probably do the math if I can get terminal velocities) as well as penetration (both over-penetration of walls as well as ability to defeat body armor). I know that one concern of the .223 is the need for a certain break point velocity in order to properly yaw and do it's magic.

Anecdotally, I am noticing 9mm uppers showing up on the market as LEO's move to .223 and that is part of what has piqued my interest.

I have tons of time on this, as funding dictates reality.

Any help pointing in info directions would be useful!

 

[rhino]

If you stick with longer OTM bullets such as 68, 75, or 77gr you should get excellent terminal ballistics at ranges we're likely to need it. Obviously you lose muzzle velocity with the shorty barrel, but the longer the projectile is, the more likely it will yaw upon impact and subsequently fragment (both of which are Good Things).

If you're going to use 55gr FMJ like M193-spec, your effective range for the same effectsi is going to be significantly less. I haven't looked at the numbers recently enough to give you distances in terms of yards/meters for reliable fragmentation, but the reduction is significant.

If it's going to be a house gun with a suppressor, use something like Hornady TAP 75gr OTM and you'll have a real winner. A 9mm carbine isn't in the same league.

For centerfire rifle calibers, energy is a significant concern. For pistol calibers including those fired from carbines, it's not significant because the projectiles are too slow to create the same kind of dramatic temporary wound cavity like you'll see with rifles. So your wounding mechanism is going to be how much damage to tissues and/or the central nervous system you can do as the bullet cuts/passes through stuff.

If it's going to be a defensive tool, I'd go with the 5.56 unless you need to be ultra, ultra short even with a suppressor.

 

[techres]

If it's going to be a defensive tool, I'd go with the 5.56 unless you need to be ultra, ultra short even with a suppressor.

I am doubtful about a suppressor. When I get a can, it will be for something that can truly be silent (like a P22). And, yes, my main defensive round is 75 grain boat tail (usually TAP).

If you have any idea about where I can find "the numbers", I would love a link.

 

[rhino]

I've seen charts on AR15.COM. I think maybe ammo oracle, or in the ammunition forum.

 

[Slow Hand]

If it's going to be a house gun with a suppressor, use something like Hornady TAP 75gr OTM and you'll have a real winner. A 9mm carbine isn't in the same league.

Isn't there concern with short barrel and baffle strikes? Soemthing about th ebullet being stabilized in short barrels? I would assume it would be worse with the heavier bullet, but I don't know.

Doug K

 

[rhino]

Isn't there concern with short barrel and baffle strikes? Soemthing about th ebullet being stabilized in short barrels? I would assume it would be worse with the heavier bullet, but I don't know.

Doug K

Hmm ... I've never read or heard anything about that, but that doesn't mean it's not true.

I would expect hits on the baffles to be caused by the suppressor not being aligned perfectly with the boreline, though.

I don't see why a bullet would not be stabilized in a short barrel (like 11.5 inches) as long as it had an appropriate rate of twist. And even if it were not properly stabilized, I would not expect significant deviation from the direction of the boreline in the few inches the bullet will travel through the suppressor.

But I'm just thinking and I'm far from "knowing" on this subject.

 

[Slow Hand]

that's waht i kind of thought, too. But, I've never played with the .223 in anything like a Contender, so under 16" is a mystery to me. I'll be learnign a bit about 13" barrels and .223 in a few months. Will let you know how that works out...

I've read on ARFCOM (take that for what it's worth) that most supressor manufacturers will not warranty any can on a gun under 12 or so inches. I'm sure Craig knows more about this than I ever will, and hopefully he'll chime in as to whether this is an internet rumor or not.

Doug K

 

[bigcraig]

To answer techres question, a 9mm AR is a range toy that offers you the same manual of arms that your "go-to" gun should be, a 5.56 AR. Stay with the intended AR caliber and use the heavy projectiles, the 75gr TAP is a great round for SBRs.

As far as the suppressor issue, Doug, I know that all, but one, suppressor mfg that will not warranty the use of their suppressor if the barrel used is shorter than 10.5". (Ops Inc. is the exception but they ask that you send them the upper to install the mounting collar and brake.)

The reasons that most mfgs will not warranty use on shorter barrel lengths:

1) Under stabilization of rounds, I have heard this arguement for years yet have never really get a grasp on the reasoning. If your chosen round will stabilize in a 7.5" barrel then you should be good to go.

2) Blast baffle errosion, unburnt powder that is prevelant when firing high power centerfire rifle rounds that are designed to for longer barrels, has the effect of sandblasting the blast baffle.

3) Baffle errosion, due to the build-up and detonation of unburned powder inside the suppressor body itself.

4) Correct threading of the muzzle, the muzzle threads must be concentric to the bore, this very important no matter what length the barrel is, but is more so with the shorter barrels as you have a shorter distance to "stabilize" the round.

That being said, I know of few folks that have run suppressors on 7.5" barreled ARs with good success, however, the kept the FA fire very limited.

 

[JosephR]

55 grain will stabilize fine out of shorter barrels. Heavier or longer projos need a faster twist rate. Where 1:9 is fine for a 55, 1:7 is recommended or necessary for 62-75.

I have an 11.5" 1:7 that I use with my Gemtech G5. No issues yet, knock on wood!

I HAVE fired enough fast enough to cause concern though. After my last range trip, I noticed a LOT of fouling on my G5 mount. After scrubbing and soaking, I found out it was molten copper. NOT a good thing. It means the jackets are getting soft before exiting the barrel. If the jackets are softening then the lead will soften too and probably destabilize the round. Luckily I had no baffle strikes.

I'm not worried- it was all semi auto with a few three round bursts from bumping. The Gemtech G5 can be repaired and I'm sure Gemtech would be happy to fix the problem after seeing my semi lower and 11.5" 1:7 BCM barrel.