Nice option but it must not have been all that popular, all the C-Lect choke Mossbergs I see are 40 years old.If you haven't bought it yet just check things that you would check on a more recent gun, but look a little closer(for things like rust or obvious excessive wear).), both of which sold many brand labeled guns made by companies like Mossberg.The Mossberg 800 came in different calibers which were designated by a letter following the model number.You'll have better luck with this request if you make it clear that you plan to buy another barrel from that shop and want to make sure it will fit.Asking to try a used barrel doesn't risk scratching a new, never installed one. The original barrel is a mile long, killed thousands of birds with that girl before some internal parts broke. I bought an over under for shooting clays, and use it now.
I think current barrels will still fit, ask a gunshop if they have a used, current style 20 gauge 500 you can try to swap barrels with.
I have a Western Auto 12ga that is old and has no serial number. The barrel should be marked as to what choke system was installed at the factory.
Asking to try a used barrel doesn't risk scratching a new, never installed one. Not as common on a 500, but not rare enough to make the gun bring a higher price either.
The 800A, the most common, was chambered in .308 Winchester, the 800B in .243 and the 800C in .22-250.
These were standard grades, Mossberg also made a higher end version with Mannlicher style stocks and factory mounted scopes. I will polish the bolt and oil the internals and wax the external surfaces metal surfaces as well as the internal & external wood.