I'm going to be participating in Humans vs Zombies for the first time, and I've never modded before but willing to try.  What gun should I get? My price limit is ~$50, and I'm curious what kind of mods are easy for first-timers out there.  If possible, I would like to purchase a gun with a shotgun cocking mechanism (pull the bolt back with the grip, not something on top of the gun).

Any suggestions?