Heck, just get the 12ga slug bbl, switch it up when you need to hunt birds! The 20 will do fine on birds if you dont want to take the slug bbl off anyways.

Something no one has mentioned:
While I had a Moss 500 rifled barrel as my deer gun for years, I wanted to make it my dedicated deer gun, mounted a dovetail on the receiver, took it to the gun smith to have that nasty 10 pound trigger worked,,, can't be done, and nobody makes triggers for the moss 500.

Trust me, you WILL out grow that trigger on the Moss 500, went with a Rem as my dedicated deer gun, and had a "drop in" 2-1/4# trigger put in.

Slack