If I ever had a "Signature" Issuse, this would be it. YES, PaP A 2ND TIME!!! I have quit zombies...for the most part, due to reaching lvl 40 and beyond and finding myself incapable of killing zombies. Sure I can do Rtrains and make them pass through a trap, but what's the point. i don't feel any sense of satisfaction. If I wanted to run zombies through traps I could do that as soon as I turn on the power.
A small spiel: Traps costs money to kill zombies, don't give any points in return, don't count toward your kill count, and can kill any zombie any time. If I followed the logic of traps, I would care more about having the 1k to turn it on than care what Gun I have. By that logic I Should start using the trap around lvl 25.
As anyone knows, Nazi zombies are about killing zombies and getting better guns with the points earned. PaPX2 (My Trademark
) is the natural evolution of the PaP in Der Reise.
As to the question of buying upgraded ammo for PaP or PaPX2 weapons...I believe you should be able to buy Max ammo. Hear me out! Think about a difficult perk to buy that entails great risk, but an even better reward. Jugg. on Der Reise comes to mind. Now, think about a dead end that had a machine that would reload all of your ammo for 5k. If you had Monkey Bombs or Gersch, the cost would be 7.5k. The Real magic would have to be in the design of the map. Create a good balance between going for Max Ammo and having to fight your way back out of the hole you are in. A simple trick, just off the top of my head, would be to create a narrow corridor to a basement area, not necessarily an "actual" basement. This basement would funnel zombies towards the Max Ammo machine. If you really wanted to make it tricky, make the corridor the width of going from the Stage to the Lobby on Kino, and the Room would be no wider...just longer. Scary!!!
Now, No cheating! No traps, no fire pits...just the guns in your hands. If you want a Real death trap, put a window in at the back. Too hard?
Another idea, make the Max Ammo take longer. It could be something like the PaP where the machine takes your gun, spins it around, then gives it back with full ammo. If you have Monkey Bombs or Gersch...it would take even longer. Maybe make it where you HAVE to refilll both your guns, THEN you can get the Monkey Bombs or Gersch for 2.5k. Don't let people through their monkeys from the inside of the room, or corridor, to the outside. Too easy, if you can just refill Monkeys and through them again. The balance would be that zombies would coagulate around the small corridor, lessening the effects of Monkeys. Explosives would also lessen because the closest portion of the zombies would take the Most damage while the back would receive the least. All the while more zombies are pouring in. Lol! Plus, you have NO IDEA how many zombies are behind the lead zombies.
This is turning in to THE TUNNEL OF DOOM!!! With Max Ammo! 
Happy Hunting! 