Get yourself a rifled choke tube and shoot sabots. I've used this set-up to shoot Winchester 1 oz. sabots and had very good and dependable results...but not for ranges beyond 60-70 yards. You might be able to reach further...I just dont know because I have not tried.

Good luck

My Berretta smooth bore groups very nicely at 75 yards using Brenneke Rotweilers.

Have you tried e-mailing Mossberg about what choke to use?

I've shot just about every slug out there with a bunch of differant guns and barrels. The one thing I've learned is about every gun likes a differant slug. If you want to get the best patterns, buy a variety of slugs and shoot them, one will shine!

A couple things I've learned, typically 2 3/4" slugs tend to pattern better than 3" and several times the cheapest slugs pattern the best, why I don't know!

Good Luck!