This is a glitch on Fallout New Vegas where the player can kill somebody and not lose any karma or gaining infamy.

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1) Clear your system cache.
2) Get an "Anti-Material rifle"
3) Buy or find "Incendiary Ammunition" for the weapon.
4) Equip the weapon and change the ammo type to incendiary.
5) Use VATS on a target and quit the VATS after targeting.
6) The target will set on fire and this will not cause anybody else to attack you.
7) This can be used several times on the same enemy.

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Enjoy gaining access to places for free and looting enemies and allies.

Note: You can use this method to gain NCR Ranger Combat Armor and stay allied with the NCR.

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If you are having any problems, just watch this video tutorial: