New at SHOT Show: Ten-X Tactical TX-12 Special Task Weapon - Breech fed single shot 12 gauge that attached to bottom rail of your AR. Fire the shotgun with your weak hand, rifle with the strong hand.
Ha! I was going to answer "why not?" too but they are aimed at law enforcement and military for breaking doors down without need for a separate breaching shotgun. Just one thing to carry :-)
if I had the need to breach I think I would just carry an extra weapon, this device is single shot, how long is it gonna take to reload that bad boy?? By adding this you kinda loose the eliment of supprise, or have 4-5 people breaching a single door.
They've had these in different designs. Better known to older Army guys as a "Master Key".