A friend of mine shoots an 870 with a Hastings barrel. He had to tighten the barrel nut with a pair of Channel locks to get it to shoot well. I'm not recommending this method, but you should make sure it is tight.

I can get 3" groups at 50 with a Mossberg 500 smoothbore/rifle sights and rifled slugs no problem. So, I suspect you can do better, just need to find the right slug. I'm shooting Brenneke KO Sabots (not the hybrid style, the sabot version of the KO) from my Marlin 512 and I can get1-2" at 50.