Safety Harbor Firearms manufacture the Kompact Entry Gun (KEG) short barrel pistol-gripped shotguns build upon Mossberg 500 and Remington 870 receivers. Their guns are classed as AOW (All Other Weapons) and so do not carry the hefty $200 stamp required to purchase Short Barreled Shotguns in the USA. An AOW stamp is $5.
Because they are classified as AOW, they can only be built using shotguns that originally had pistol grips. Remington only make 12 gauge shotguns with pistol grips, so if you want a Kompact Entry Gun built on a Remington 870 receiver you can only have it in 12 gauge. Mossberg manufactures .410, 20 and 12 gauge shotguns with pistol grips and so all calibers are available on Mossy receivers.
I asked the company who their primary market is, expecting them to say law enforcement, for breaching purposes, but was told that their biggest market was enthusiastic shooters who want something a little different. The shotgun pictured at the top of this post is a .410 chambered KEG with a 2+1 capacity, which must be a really fun plinker.