I found a bug...
1) Equip hunting rifle as primary & chainsaw as secondary.
2) Switch to chainsaw.
3) Wait until you have a bunch of zombies at the wall, then use the chainsaw on them.
4) Switch to rifle whilst using the chainsaw.
5) Rifle now has 98 ammo.
It doesn't always happen, but if you time it right it's pretty consistent.
 "Boxhead: 2Play Rooms"){kind=icon}
 "The Last Stand"){kind=icon}
 "Kongai Launch"){kind=icon}
 "Boxhead: The Zombie Wars"){kind=icon}