1) There is a limit to how hot you can load a 5.56mm.
2) Barrel length is crucial to speed, there's no getting arund that.
3) Short barreled, hot loaded 5.56mm weapons are NOT multi-hundred meter target guns, plain and simple.
I don't own such, but I have fired, chrono'd, and tested them. When they were built well, the accuracy was suprisingly good.
However, the reliability of 10" and 11.5" barreled CAR's was far below that of 16" and 20" AR-15/M-16/CAR's. The two most reliable types of shorty .223's that I have ever fired were HK G36K's and Ruger AC-556K's.
You often speak of "cans" (suppressors) in conjunction with CAR's. With an unsuppressed shorty .223, you need a suppressor. The muzzle blast is painful, nearly unbearable. The trouble is that suppressors add length and some weight. A suppressed shorty may have military merit, and it sure is fun!
If you really had the experience with the shorty CAR's that you claim, you would almost certainly favor a 14.5" or 16" barrel.
Gunkid I posted the links to sites that not only had chronos but also the cut the barrel down, one inch at a time, to 10 inches a 5.56 and chronographed it! That's PROOF you are so FOS on the subject of the 5.56, velocities, and reloading.
So it is YOU WHO ARE ALL MOUTH. You can talk a talk, but cannot walk any kind of walk.