I was given a 12 gauge Smith & Wesson pump shotgun that is seized up from being stored in high humidity No chokes, just full choke ribbed barrel. Any suggestions on what to start with to clean it up?

It's most likely locked up from rust or something. I'd start with Kroil, or a good penetrating oil. I'd spray/soak it all around the chamber, action, and anything that moves. Let is sit for 1-2 days, and check it again. Just clean it up as good as possible.

Thank you, I have never heard of Kroil.

What Chad said. Kroil is good stuff, also known as the oil that creeps. An excellent penetrant.

If that doesn't work the next step I would take is if you can take off the buttstock and forend, dunk the barreled action in a metal bucket filled with enough diesel fuel to submerge the action. Check in 1-2 days.

It goes with out saying that an open metal bucket half filled with diesel fuel is a fire hazard, please keep in a safe location away from fire or flame, etc.

I had to do this to get an old RCBS reloading press ram loose once upon a time.

Just my .02,
LeonCarr

Ive read vinegar is good for cleaning up rust... not sure what it will do to any finish on the gun.

Yup. PB Blaster is a good one as well.

Thanks for all of the advice. Got the barrel off, got iall of the actions moving. Thinking I will take steel wool to all exposed metal, prime it and camo paint it. However, got to thinking about the barrel. How do you recommend polishing the inside of that before putting rounds thru it?

Shotgun rod
Drill
New mop end of proper gauge
JB bore compound

Makes for a mirror finish

Vinegar with strip every bit of bluing off a firearm and rust will set in immediately.

Thanks for the info, Maximum. The bluing is already gone, so vinegar might be the way to go to get all of the rust off. Could I prime it immediately afterwards, like other metals? Since I am going to paint it, it seems logical.