Purchase or find the Anti-Material Rifle. Then, equip it with incendiary rounds. Target an enemy or enemies with it in VATS, then close VATS. You will not use any ammunition, but they will still be on fire and lose health. You will not aggro enemies or friendlies. Also, you will not lose Karma or Reputation with each kill. This glitch is very useful if you need to kill a NCR/Legion that has a weapon or item on them. Note: This glitch was done on an unpatched version of the game.