Just to dig up the shot gun on firearm. You can actually use madbull shot shell in fireearms. The down side is you will probably want to make your own firing pin as the firearm one is made to dent the primer (gas release valve in this case)
Never the less, if you intend on fielding it, remind your self that is a firearm, not an airsoft. you must follow all regulation of that including no discharge in city.