Go to the Mainframe on Der Riese.

You have to be able to upgrade your weapons and have to have Bouncing Bettys.

Go to upgrade your weapon, upgrade it and while its upgrading go buy a Kar 98K, then go back to get your weapon and pull out a Bouncing Betty.

Right then push X, if it dosent work tap it again shortly after.

It should leave you with a third weapon. You can trade your Kar 98K for a weapon of your choice. Either trade it in the box or trade it for something off the wall.