Built this rack today for my AR-15. Would work with other compact long guns and I already have a second design in my head that will be more flexible for different size weapons.
Holds two 16'' AR's or similar weapons (or any object that will fit in the mount). Benelli M2 would not fit (handguard extends too high), but I will build another version of the rack that will have some adjustability.
Mounts to the headrest, goes on and off in seconds.