If you shoot an NPC in the game, he and his faction will open fire on you. However, you can use the red explosive barrels to kill them and it will not count as a "source of damage" against your character.

You can try this against some of the Freedom NPCs in the Army Warehouses who are standing next to what amounts to a giant bomb that only needs your gunfire to detonate. In fact, if you have the patience to locate red explosive barrels in pacified areas, you can use this exploit to bag a lot of items and loot.

Submitted by Taylor Forestman (Virginia Tech)