In a perfect world, all entry rifles are short and powerful. Within reason, the shorter the better. Kitted up in 50 pounds or more of gear, rifle plates and a gas mask, using long barrels can be just, well, inconvenient to wield in the urban sprawl that makes up most law enforcement environments. Get behind the wheel of a vehicle and it is even worse. There is nothing like wielding an 8-inch-barreled rifle in a car, house or close-quarters structure. An added benefit would be the ability to suppress these short rifles without making them too long. Add a suppressor to an 8-inch barrel and, worst case, you are back to M4 Carbine lengths.
In the law enforcement arena, the ranges are almost always across the room, at most across the street, so long-range effectiveness is really not an issue. An 8-inch-barreled 5.56mm will do just about anything most SWAT teams will ever need, especially on entry detail. The need and value has never been the problem—it has all been in the application.
Almost without regard to caliber, truly short-barreled AR rifles have always been somewhat problematic. Before the advent of the piston-driven AR rifle, this was pretty well true, but even the piston-driven ones have their issues. A piston gun will run cleaner, but depending on the suppressor, the bolt speed is increased so much that they can become completely unreliable. You can try to run your short gun without a “can” (suppressor), but they are ear-splittingly loud. They can take you right out of the fight. Suffice it to say, producing guns this size that work can be a headache.