A couple of years ago my wife won a Mossberg 500 w/ 24" ported barrell at an NWTF event and I have never owned a Mossberg or thought to highly of them (Grew up on an 870). But decides to experiment with it. So, I put a .660 Primos Jellyhead and shot 3" Hevi13 #7's. Also, put a a set of gamereaper mounts w/ a Nikon BTR Turkey scope. This gun far exceeded my expectations. Shot in the high 200's at 40 yds. Granted, I believe this has alot to do with the LOT # from Hevi-Shot. But, the potential is there with good shells.
On a side note: If you plan to scope a Mossberg 500 try to avoid gamereaper mounts. Dont get me wrong. I love gamereapers. But the scope will sit over the thumb safety on the comb and trust me the gun will go out of safe just fine but getting it back on safe is another story. Especially when your thumbs resemble plump sausage links, as mine do!
Hope this helps!