In career mode. It has a baseline inventory. Weapons from a mod may not be in that inventory for several reasons. i.e., intro date of the weapon, and others. I got around this using the "MW5 Save Editor Verson 1" utility mod. After your manual save, Not an AUTO SAVE, log out of MW5. Open the Save Editor. You can add weapons and associated ammo to that save. So, when you open that save again to edit your mech, the weapon and ammo will be there to install. Don't forget to add the ammo. I hope this helps. Good Luck.