Find a gunsmith and ask for help.
Follow owner's manual instructions.
Contact the maker if you don't have a manual.
Clean it.
The owner's manual provides instructions for disassembly to the extent necessary to properly clean the shotgun. Replacement owner's manuals are available on the Remington web site.
18 AND CLEAN RECORD
unless it wont shoot than no
Ask a gun shop or gunsmith for assistance.
Ask a gunsmith for help.
Take it apart, clean it, lubricate it and put it back together.
Recommend you ask a gunsmith for help.
To clean a Western Field 12 gauge pump shotgun M550AD, first ensure the firearm is unloaded. Disassemble the shotgun by removing the forend and barrel from the receiver, following the manufacturer's instructions. Use a cleaning rod and appropriate brush to clean the barrel, followed by a patch soaked in gun solvent to remove any residue. Wipe down the receiver and other parts with a clean cloth, lubricate moving parts with gun oil, and reassemble the shotgun.
ABOUT $50 TO $75 IF VERY CLEAN
http://www.histandard.info/manuals/sears/pashotguns/index.html
Best to ask a gunsmith for help