You didn`t mention if this is a new gun, etc. Anyway, if you haven`t done the following do so before trying anything more drastic:
Field strip the gun per owners manual and clean thoroughly. I like Simple green, others have other preferences.
Varify the the magazine tube is screwed in right. On a new gun this should be a non-issue because they are `glued`in at the factory.
Lub with any name brand gun oil. I like Break Free, others have other preferences. Lubing means putting the THINNEST LAYER OF OIL HUMANLY POSSIBLE and wiping off any excess.
Make sure the barrel is seated correctly. This done by turning the barrel nut firmly but with hand pressure only.
You also didn`t mention if there are any aftermarket parts or if a previous owner was an amateur gunsmith. If either of these is true, look at everything with a very suspicious eye.
Owners manual is available a Mossberg.com and a field stripping guide is also posted on this site. Let us know what eventually works out.