Saw a really cool stand off device.

I saw a neat variation of a door breaching device. Rather than having the flash hider thing on the muzzle, this shotgun had a magazine tube that extended about one shell length longer than the barrel. The "crown" was attached to the extension not the barrel. It could easily be removed and a shorter extension installed when not needed. It looked like an 18" barrel with an extension designed for a 20" with the wood grabbing crown welded on. I have no idea if this was factory or home made.