Are all 500s built on the same size receiver or are the 20ga and .410 built on a smaller receiver??? I've never owned anything other than their 12ga but thinking about a 20ga but not if it's built on the same receiver as the 12ga.
There is a slight variation in width between 12ga and 20ga receivers.
Most 12ga stocks and accessories will fit on the 20ga receivers though. With the stocks there will be a little overlap since the receiver is a tad bit more narrow, but several folks around here have mounted 12ga stocks to their 20ga shotguns with no issues...