I’m surprised 9mm ARs are not more popular considering the prices of 5.56/.223 ammo in recent times.
Is there 9mm, expanding ammo with thicker jackets to slow expansion due to higher velocities through a carbine?
Have you tried to find 9mm online recently?
223/556 is a lot easier to find around here than 9mm. Walmart yesterday had an American Eagle 1000pk of .223, and right at 100, 20 round boxes of PPU on one shelf and right at 50 boxes of Tula on another shelf. Zero pistol calibers anywhere. The PPU was running 7 bucks a box and the Tula was running 5 bucks a box.
I built a 9mm carbine about 10 years ago to take advantage of the much cheaper ammunition and to be able to shoot it on local indoor ranges in the Winter (handgun calibers have a much lower range fee around here.) It hasn’t gotten shot much in recent years because of the poor availability of cheap 115g RN.
So… I built a dedicated .22LR AR a few years back to take up the slack. You can see how that’s worked out for me recently. :rolleyes:
Full sympathy, Heavy D. I feel your pain; been there, done that. 
At the risk of stating the obvious, there are advantages to the 9mm regarding where and what you can shoot with it. A buddy and I whack steel plates with ours inside 50 yards; no cratering or backsplash. Nines remain cheap and easy to reload, and they’ll work with lead bullets.
For the OP, I’m not sure how much of a velocity increase the 16" (or 10.5" in my case) may actually produce; there should be some, but the burn rate in 9mm rounds is optimized for handguns.
All we can hope is that the current goofiness soon subsides regarding ammo.
Moon
9MM does come out hotter from longer barrels to a point, but after the first 6", its at most 30 FPS per inch, oftentimes however much less, in the 10-15 FPS per inch range.
I like my 9MM carbine, well SBR (5.5" barrel but it always has a 8" suppressor on it). Fun, quiet, reliable and accurate.
Thanks for the replies, all.
I bought a 9 mm AR15 when they first came out. Mine has a standard size ejection port cover, not the short one that is on some of the newer 9 mm models. My bolt is a semi auto only and only works a short, 9 mm semi auto hammer. I’ve seen some of the newer ones that have the full auto bolt with a hole in the top to clear a standard size hammer. My 9 mm bolt has no hole in the top. The newer ones have the stronger silver hammer pin too as the 9 mm is harder on the hammer pin. The early 9 mm buffer is made of two pieces and the later ones are one piece construction. The two piece one is usually reliable in 5.56 rifles where the one piece one sometimes won’t work well with 5.56. They are the same weight but for some reason the early 9 mm buffer works better with 5.56.