Ok, I can understand it was a home made gun but how did he figure out that the guy had 4 rounds or shells on him?
Was the gun loaded?
Luckily for the off duty cop this idiot didn't open up on him. A shotgun can be very nasty at close range.
I expect one the perp was disarmed and cuffed was when the off duty cop found the shells.
Nutters in every country....