I too would recommend against an AR style pistol grip stock on a M500, it does make the controls very difficult to manipulate from a shooting position (AR style stocks are useable on a Remington 870 however, IMHO).
I bought one a couple of years ago and put it on one of my M500s, used it for less than a month, then removed it and gifted it to a buddy because he liked the "tactical" look.
I found it made the safety basically impossible for me to operate and the slide release was extremely difficult to engage. IMO the tang safety on a M500 is one of its best selling points, especially for a HD shotgun, as it makes the gun more manageable for a left handed shooter (like my Dad who has been a Mossberg man my whole life) and also makes it better suited to weak side shooting by a righty (like me who prefers the M500 for HD for a variety of reasons).
I also recommend trying the 12" L.O.P. stock, I have one on the M500 by my bed and would't have anything else for HD. It makes the gun very maneuverable in tight spaces, such as hallways and around corners.
Combined with an 18.5" barrel, the 12" L.O.P. stock makes the M500 a very compact weapon. Add a Surefire flashlight, and I consider the whole package to be the cats pajamas for a home defense shotgun.
Papershotshells