Learn to reload 12ga buckshot and you can get a lot more out of them.
A standard 2.75" case 12ga can actually handle 12x 00/SG pellets at full power.
Or 20 pellets of the US 's no. 1 buck. Devestating load, much heavier pellet than SSG and two extra pellets to boot.(Our SSG for the record is between US no 2 and 3 buck in sizes. No 4 buck is smaller as you know)
A 3" case ,which many shotguns chamber, can handle 15 x 00/SG.
A 3.5" case( which are available in some double barrels, I think Hatsan does an affordable one off the top of my head) can handle 18 pellets of 00/SG or 50 pellets of No.4 buck.
You also used to be able to get winchester and raytrade(remington) to do special orders of the american loaded buckshot if you order a case or more( 250 shellls) through your LGS. I have brought in factory loaded 12x00, 15x00 and 18x 00 in the past from them and the 1 buck load mentioned above.
http://www.ballsticproducts.com in the US are great to deal with and have all the components for loading.