I'll upload a few in a bit to this OP, but for now lets see what you've done? The modifications on these are endless, and they are quite fun and cheap to mod, so lets see them!
It's not a 500, but an 835. I started off with matte 20" thumbhole stock turkey gun:
I added a custom made extension and pistol grip. I also have a Spec Ops NRS for it.
Next I ordered a factory 18.5" 835 smooth bore barrel from Mossberg. It looks like any 18.5" smooth bore barrel with a bead sight but it can shoot 3.5 shells. It's also not over bored so it can shoot slugs. I can't seem to find any pictures of it at the moment, though it has been to a few shoots. Cost me way too much in the end ($700 total), but I have a zombie gun than can shoot 3.5" 18 pellet OO
Yea, that stock went to the gun storage closet pretty quickly. Good news is that any standard Mossberg stock will fit, I just haven't come across a used one for cheap enough yet.