If you buy or find a Anti-Materiel Rifle, buy one flammable bullet (can't remember the name) and enter V.A.T.S on a target. When you exit V.A.T.S, the target will ignite, and their health will decrease. You can continue to enter and exit V.A.T.S and the targets health will continue to go down. When the target eventually dies, your reputation with the faction will not be affected.

Enjoy this trick :)

You can use it on N.C.R veterans and get good gear :)