I shoot a 870 with a slug barrel. I found that faster slugs didn't group well (like the Hornady SST's) and slugs with a bit lower muzzle velocity grouped better. Ended up using Copper Solids. But, that's my gun -- yours may be different. I've also found that backing off on the amount of powder I use in my muzzleloader improved accuracy. So keep this in mind if you are using something faster and getting poor results.