870 jamming after firing

What could be happening is that something within, or at the mouth of the chamber, is causing the chamber to be tight when the shells are fired, and the brass head has expanded.  The easiest way to determine if this is the cause is to safely cycle dummy ammunition through the gun without firing.  If this ammunition does not jam up until fired, then polishing the chamber should be all that is necessary to resolve the problem.  This polishing will remove any high or abrasive spot and result in the gun to cycle smoothly and reliably as it should.  If cycling live ammunition instead of dummy rounds, please be sure to follow the Ten Commandment of Fire Arms Safety.