As Terry said, if you have a over bored barrel it is not recommended to shoot slugs throught it.
Here's a passage from the FAQ page on the Mossberg site.
"Can I fire slugs through my 935™ or 835® vent rib, Accu-Mag® choke tube barrel?
No, 935™ and 835® field barrels (those with a ventilated rib) are "overbored," and the extra inside diameter may cause the projectile to "wobble" producing unreliable accuracy. A projectile may wedge inside the barrel causing an obstruction. Obstructions of any kind can cause damage to the firearm resulting in damage to the barrel and/or personal injury to you or those around you. Dedicated fully rifled slug barrels are available for the 935™ and 835® models. Other 935™ and 835® Accu-Mag® choke tube notes: Buckshot and steel-shot loads are not recommended for use with an extra full turkey tube installed."
You can view the complete FAQ page here...
http://www.mossberg.com/content.asp?ID=508§ion=resources
You ought to print a copy of this and go back to the gun store and thumb tack it to the "gunsmiths" forehead. That guy is gonna get somebody hurt.
Oh, and by the way....
Welcome to the Realtree forums!!
hangunnr