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numrich gunparts.com has everything but the stock
Pretty much an impossible undertaking.
Parts for an American Gun Company shotgun will only be found as part of another shotgun. Haunt the local gunshows for one that is being parted-out. As for the gauge (not caliber), did you mean "bigger than a 410"? There have been 24, 28, and 32 gauge shotguns made. Of these, only the 28 is still available in the US, but I believe 32 gauge shells may still be commercially loaded in Europe.
Try gun shows, Shotgun News, Gun List, local gun shops.
You can purchase parts for a Ranger model 30 16 gauge shotgun at these locations. Gunbroker, Topgunsupply, Budsgunshop, Gunpartshop and Midwayusa.
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There are some sources for Crescent shotgun parts, but it is very possible that parts will have to be repaired/made.
DO NOT ATTEMPT TO USE PARTS THAT WERE NOT MADE FOR YOUR WEAPON!!!!!!!!!!! Danger, danger, danger.
I have parts for that model. sales@countrygunsmith.net
Try e-gunparts.com
looking for stock model sb 20 gauge
Unlikely. Try e-gunparts.com