You see why I own two M-500s? Simply because a used 500 is one of the best bargains on the used market. The resale sucks. Probably why boobooboo has kept is non fucntioning one. You can't get squat out of one that works and nothing out of one thats broke. :>)
The two I have are both 12s, one plain fixed, mod. the other had a 24" rifle sight smooth bore and 28" v.r. with tubes.
$100 for the plain jane and $125 for the two barrel job.
I use mine a test beds for developing slug loads. If I break one I'm out little.
One is my night stalker. It's the 28", sports turkey full, and wears red spot light gear for calling night preditors.
I'f it falls from a tree stand, gets dropped in the mud, it's a $125 utility gun not a big loss.
I would suggest you go to ebay and buy a new bolt, firing pin, associated springs and replace all that. I bought an extra plain bbl, fixed choke from ebay for 28 bucks. Cut it off and made a cyl. bore brush barrel for rabbits.
There are tons of Mossy parts on ebay dirt cheap.