IANAL. However, I think that would fall into the catagory of legal defense of others. But, I would not admit to doing so, as it is likely drug runners you'd be shooting at, and if they found out who you were, you and your family would likely be in danger of reprisals.
For the most part those folks want to avoid being noticed, and will usually only open fire in desperation type of situation. While hunting in 30B last november along the San Pedro near the border my hunting partner came up on his horse and told me to get on. He had seen a group of about 4 guys, all in black with backpacks hiding in the area. We figured at that point it was time to call it day so we just got out of there. We'd been warned that those guys tend to carry Uzi's and the like, so as far as we were concerned discretion was the better part of valor.