What gun are you changing it to?
If you want it to be a SCAR-L Black, per-say, you find PVMLA_SCAR BLACK.rez (Probably not the exact name.) Right click it, and click edit w/ Hex Editor. Ctrl + A, Ctrl + C, then close it. Then find PVMLA_W_M16A3.rez, right click, edit w/ hex editor. Do the same for the textures file.