If I remember correctly. Mossberg made/makes two different 500's. The only difference is the gauge. 500A being 12 gauge and 500B being 20 gauge. Also, that being said, the exterior dimensions are the same. Sorry I have a 500A but I don't have a 500B to compare it with.
To answer your question, Yes it should fit. (I believe that 590 stocks will also fit, but I have not personally seen one, so I'm not sure)