Seeing as it's going to be patched in the upcoming week or 2. (Here's the petition that was created for it. [ Register or Signin to view external links. ]
ANYWAYS onto business!
Note: This can only be done efficiently with the Destroyer as devastator mode restocks your ammo due to this glitch and the 1st and last people in the lobby CAN'T do this.
STEP 1. Get your missile launcher out.
STEP 2. Shoot one missile, keep the trigger held in and wait 3 seconds.
STEP 3. KEEP the trigger held in and switch to any explosive weapon (Striker, Falcon, Krysae or Scorpion.)
STEP 4. Wait for your gun to fire, then release the trigger and boom!
STEP 5. Enjoy while it lasts.