Browning did not offer one, but you may find an aftermarket Hastings barrel (www.hastingsbarrels.com) that will do the trick.
Try gunbroker.com in the barrel section.
No however barreta 302 and 303 is the same gun browning built the b-80 under their patients and made a slug barrel that will match up to the b-80 for around $300 plus shipping
Slug barrels were not made for auto 5's until 1962. These newer barrels will work, but need to be fitted. You may find a slug barrel for a 16ga on gunbroker.com either Belgian or Japanese, but you will need a qualified gunsmith to fit it to your gun.
No
Will be hard to find, and expensive. Do an internet search for Corson's Barrels.
Try egunparts.com, gun shop, gun show, pawn shop
Try e-gunparts.com
Check the internet auction sites, gun shops, gun shows, place an ad, try e-gunparts.com
Lacking a slug barrel, improved cylinder works best for slugs.
Try Browning, gun shops, gun shows, estate sales, for sale ads, want ad, pawn shop, garage sales.
Incomplete serial number. Value from $500-$1000.