One of the shortcomings of the AR platform is its need for a buffer system. That is true whether piston-driven or standard gas impingement. It really limits the ability to shorten the system, as a folding stock is really not possible. The collapsible stock certainly helps, but it really isn’t the same. As a general rule, this means that the barrel must be shortened in order to make the system really portable.
This presents its own issues, as most AR-platform rifles become quite problematic as the barrel is shortened. Even with piston-driven gas systems, they can become unreliable and/or ammunition-specific. They certainly work, but it is not always simple, and you are still relegated to a 7- or 8-inch barrel.
The Para TTR is a reliable and proven system in a very compact package, perfectly suited to police use. Shown here with an Aimpoint CompM4s, SureFire 60-round quad-mag and a SureFire M720V WeaponLight.
The next issue that arises is the ballistics of the 5.56mm round. From a law enforcement perspective, this in all likelihood will be the round used. There are some bigger rounds better suited to short barrels, but they are not generally in use by police departments or officers. With a really short barrel, you are getting close to .22 Mag ballistics.
The short 11” barrel of the TTR SBR places the muzzle device just forward of the handguard for maximum compactness.