"Longer barrels=higher velocities, we don't want the bullets dribbling out the end of the barrels at 800 FPS like they do on MP5SD's"
This is an issue with my LRM M169. It was designed to be used with 115 grain bullets. If you start playing around with loads you get all kinds of weird velocities. It you try to shoot actual subsonic ammo you will get velocities like 800 fps.
If I had it to do over again, I would have specified that I wanted one without the ports to slow the bullet down to subsonic velocity. I would rather tailor my ammo to subsonic velocity than have the gun do it for me. I have no control over what the gun does, I do with the ammo.
I do have a Glock pistol with a 33 round mag and an AAC Evo9 can on it and I would much rather have that than a "pistol" AR. However, it is a two stamp gun. I registered the pistol as an SBR so I can put a stock on it.