I've searched online and can't find an answer. Are the receivers of the 500A and 500C the same? Could the internals of an A be changed to C parts (along with the magazine) and basically end up with a 20 gauge pump?
I'm pretty sure the receivers/parts of the various letter models are not interchangeable. A=12ga, B=16ga (rare), C=20ga, D=28ga (never made), E=.410 bore. The reason I say this is because stocks from a 12ga will sit proud on a 20ga receiver meaning the receiver is narrower.
Remmy had older Wingmasters that could swap gauges by swapping bolt and barrel.
Be patient as I'm sure someone else will come around with more info.