if you can spare to spend $300 you can't go wrong with the Leupold VX-2, I have many of them on almost all my slug guns all in 3-9x40

I do have the Mossberg 500 in 20ga as a slug gun as well, I don't use it at all I have it for family/friends to use and it shoots very well, it has on top a Bushnell Trophy XLT with the plex also in 3-9x40, for the money it's a really a decent scope

http://www.walmart.com/ip/Bushnell-T...ndingMethod=rr