I'd use higher velocity rounds since it's semi automatic, to make sure it completely cycles the bolt. Other than that you can use any 2-3/4 inch or 3 inch shells, bird shot, buck shot or slugs. Since it's a smooth bore Barrel you will get best results with rifled slugs. Hope this helps.

Keith, start out with high velocity loads for break in with the heavy piston. After the break in, install the light load piston and try a few boxes of lighter loads to see which will run reliably. Then you should stock up on 3 inch or 2 3/4 inch loads that you know to work. Be sure and clean after initial break in and lube the bolt and piston. Cheers and enjoy the Citadel Boss!

Keith, you can use 2 3/4" or 3" loads 1 oz or greater. You need to use at least 1300fps with at least a 1 oz load as it needs to build pressure in the barrel. If using 1500fps or greater, be sure to use the high pressure gas piston. Below 1500fps, you can use the low pressure gas piston. The reason for the fps suggestion is for the gun to generate enough gas to cycle correctly. Break in your shotgun with 50 rounds 1450-1500 fps. After break in, ammo with at least 1300fps will give best cycling results. Have a great day!