Slug gun build problems?

Not actually a "BUILD" but I put it together. I hunt hogs on some public land and where I hunt you can hunt with a shot gun, slugs ONLY and it has to be a smooth bore barrel using riffled slugs. I don't understand the rule, I just follow it. Anyway, I bought a Mossberg pump shotty, had a weaver rail mounted to it and bought a brand new Nikon Prostaff Slugun scope. Took it to Bass Pro and had it bore sited then out to the field for some fine tuning. ....At 50yards its all over the freakin place. Consistently inconsistent. Bull, then off the map, 2 in high then 9 inches low. Not sure weather I should abort this mission or if there is something I can do to get decent groups with this thing. THOUGHTS OPINIONS INPUT

Everything tight?

I agree with the above post. Sounds like loose scope mounts and/or bases. Another thought, you're not shooting sabots through that smooth bore, are you? You mentioned rifled slugs, but not what type. Good luck with it. I hope you work it out.