This uses the same technique as the fully automatic fal glitch.
Step 1: Create the classes
Originally posted by another user
M16A4 grenade launcher (fall camo)
USP.45 FMJ
Semtex
Marathon
Stopping power
Ninja
M16A4 grenade launcher and RDS (arctic camo)
USP.45 FMJ
Semtex
Bling
Stopping power
Ninja
Step 2: Make a private match - Rust
Step 3: Choose class 1 and fire a burst
Step 4: Click start and choose class 2
Step 5: Throw the semtex on the ground in front of you, killing yourself
Step 6: Go over to where you killed yourself and swap to the USP.45 FMJ
Step 7: Pick up the M16A4 grenade launcher.
Step 8: Press left on the d-pad and fire the grenades
Step 9: The camo should change colour
Step 10: Pick up your USP.45 FMJ instead of the M16A4 with the grenade launcher and RDS
Step 11: Swap to the M16A4 grenade launcher, it should be out of ammo
Step 12: Let it reload and the press left on the d-pad
Step 13: Hold down R1