I am building a 7.5 inch AOW AR.

It shall have a nasty fireball and boom to go with that short barrel, but hey, thats part of the fun!

I am only gonna get 2200 or so FPS from my snub nosed AR.  Handy-Dandy bullet fragmentation needs 2700 fps to work reliably, so I need a different approach.

I am leaning towards the 77 gr Black hills .223, since it is less dependant on velocity for break-up...but that stuff is 50 cents per round.

Maybe I should go with a 7.62x49?
Any input would be appreciated!

I went to WV last weekend, and screwed around with a couple of rounds. I took my 11.5" w/MOUT suppressor, fired (2) 55gr Speer SP handloads into a drum of water (from an upstairs window, straight into the water), the bullets both mushroomed out, 1 of them had the lead core fall out shortly after retreival. I also fired (1) Hornady 52gr HP handload into the drum--- the bullet literally exploded into many small pieces of jacket and lead.  

Distance from muzzle to water was maybe 6 feet.

Also fired some 9mm HP loads, LE Black Talon  totally expanded to the point the core fell out, Federal Hydra-Shock exploded into many pieces, Remington 124 Gold Saber did not do much, a Winchester 147gr HP mushroomed nicely (copper jacket, not a Silver-Tip). Firearm used was my 10' Uzi w/AWC suppressor.

Maybe I should go pistol carbine caliber for this short of a barrel?

Use varmint ammo.  Most of the big ammo companies sell ammo with varmint bullets.  If you do not know about varmint ammo, post again and I will explain more.

The varmint ammo violently fragments (explodes) even at low velocity.  Varmint ammo will violently fragment even as low as 2000 fps, and maybe even alot lower (I am not sure how low velocity).

Bingo.

You get adequte fragmentation for barrels as short as 9" with 75 and 77 grain rounds.  I am unsure if a 7.5" barrel would provide the same fragmentation.

Looking for more info on the varmint rounds...checked into pistol calibers, and it is cost prohibitive at this time.

the 77 grainers I can get for 50 cents per rounds, remanufactured, or 60 cents per round new.

What kind of varmint bullets do you recommend?  And, how do they fragment at such low velocities?
I saw an East German 7.62 that would fragment at like...10fps.  Not really, but it came close.  Nasty boys those rounds are.  Cant seem to findem for the 5.56.

What about the Winchester PowerPoint, or other of the like?

It is a .223 bullet that is designed to expand.  I don't know what the expansion threshhold is for the PowerPoints, but is is probably fairly low.

Winchester 64gr. Power Point Plus

The PP+ will still have worse terminal effects than 77 or 75 grain ammo out of a short barrel so long as the OTM rounds fragment.

thanks,

Looked up the reports on the PP+.  Nice

I will use a clipfull of Winchester Power Point Plus for my first response.

Thinking of having a clipfull (or 2) of heavy penetrator with every 4th round a tracer as a backup in case I be forced to shoot at a flesh eating zombie driving an auto and wearing ballistic armor.

thanks!