1. Remington with models like the 870 pump action and the 1100 series of semi automatic shotguns........they sold millions of them!
2. Air Arms and in particular their S400 series of airguns have sold like hotcakes all over the world
3. Beretta have been very successful with their entire range of sporting shotguns including the Silver pigeon and the 391 series of semi automatic shotguns.
4. Weihrauch got the HW100 multi shot PCP right very early on and it continues to sell in big numbers.
5. Mossberg have enjoyed great success with their model 500 series of pump action shotguns with its only real competitor being the Remington 870.
6. Browning has almost certainly got the crown when it comes to successful gun designs, most of which we cannot purchase over here. The Browning Auto 5 was the first mass produced semi automatic shotgun and they sold a lot of those!