You can't add an extension onto a 500 mag like you can with most pumps. The threads for the barrel screw get in the way. In order to increase capacity you have to install an longer magazine and a special barrel that has the mounting lug moved forward to fit the long mag.
Short answer is, you can't add an extension to the typical M500 since the magazine tube is closed on the end save for the hole into which the barrel nut screws.
In the relatively unlikely event your M500 is actually a MILS gun (with a M590 style magazine tube) then it's possible to add a magazine extension. AFAIK Choate made the original mag extensions used by Mossberg on the M930 semiautos... they make a decent product... http://www.riflestock.com/store/do/category/MXT/37.
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