This shotgun conversion for AR rifles was designed and built by Jonathan Ciener, well-known for the excellence of his .22 caliber AR conversions. These are no longer made, and very difficult to find.
The shotgun mounts at two points: at the bayonet lug and at the front pivot pin of the platform rifle. Note that this conversion will fit only preban rifles with 20" barrels.
I have two of these available. $849.00 each.