Step 1) On your computer, download the mod weapons that you want to put into GunApp
Step 2) Open iTunes, and click on your iPod
Step 3) Click on the 'apps' tab
Step 4) Scroll to the bottom of the page
Step 5) Click on GunApp
Step 6) Drag all the .zip files of the weapons you wanted into the section on the right
Step 7) RESTART GUNAPP and ENJOY the mod weapons!! :D
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Here is another tutorial on youtube:
NOTE: there is no known way to install mod weapons only from your device. You have to connect your device to your computer. From your PC, you drop the mod weapons into GunApp using iTunes.