1) Load the zombie map "Der Reise".
2) Play through the game until you open the Pack-a-Punch machine.
3) Earn enough money to be able to use the Pack-a-Punch machine and buy bouncing betties.
4) Put the gun you want to Pack-a-Punch into the Pack-a-Punch machine.
5) Run and buy the KAR-98 from the wall.
6) Run back to the machine and bring out your bouncing betties.
7) When it offers you to take your gun from the Pack-a-Punch machine, lower your bouncing betties and hold X as quickly as possible after.
8) You should of kept your other weapon, the Pack-a-Punched weapon and your KAR-98 which can be swapped for a better weapon later.
9) Assuming you have 3 or more guns, put a gun into the Pack-a-Punch machine, when it comes up do the same Betties trick, start to bring it up and then grab the gun from the machine.
10) Enjoy the variety of weapons.

Notes: If you hit X too late, it won't trade with the Pack-A-Punch machine.
If you do not take out the betty when you have more than three guns and switch weapon, you will lose all your weapons except two.

Here is a video tutorial: