Value difference between mossberg 500 and mossberg 500a?

Answer 1

There is no difference. A 500 is a basic Mossy 500 receiver shotgun. Unless the gauge is stated, it's assumed it's a 12. The 500A is simply stating it as a "12 gauge" 500 in its name. The Mossy 500A is a 12 gauge model 500, the 500B is a 16 gauge model 500, the 500C is a 20 gauge etc. There was no 28 gauge so there is no 500D, contrary to many sources, so the next one was the .410 called the 500E.

A 12 gauge model 500 and a 500A are the same gun.