The last deer i shot with a shotgun only needed 1 shot to surrender.
But if the mag extension is for HD then that makes sense. Most mag extension kits thread onto the forend of the mag tube, replacing the cap/nut. That makes them work with any barrel that fits your gun in its factory configuration.......if you don't want to pay out extra $ go to the local trap range and find someone with a Mossberg. Offer a trade straight across. You might get some funny looks, but I would make that trade in a heartbeat. Both of you will be happy. (And bigjim will rest easy knowing that a new Browning lover is born)