Phrog, having hashed this out in International Law for the NWC, this is not true. I am pretty sure that if someone is shooting at me, I am not putting down my weapon anyway. Rather than me go and find a law that says I can, why don't you cite the law that says you can't. (I feel silly calling a Marine out on this)
OK, I had to do a little digging because it's been a while since I looked at it. It's called
proportionality:
Proportionality. Proportionality prohibits the use of any kind or degree of force that exceeds that needed to accomplish the military objective. Proportionality compares the military advantage gained to the harm inflicted while gaining this advantage. Proportionality requires a balancing test between the concrete and direct military advantage anticipated by attacking a legitimate military target and the expected incidental civilian injury or damage. Under this balancing test, excessive incidental losses are prohibited. Proportionality seeks to prevent an attack in situations where civilian casualties would clearly outweigh military gains. This principle encourages combat forces to minimize collateral damage—the incidental, unintended destruction that occurs as a result of a lawful attack against a legitimate military target.
In this case the M-16 (which I wouldn't put down either) is proportional to the threat. If you decide NOT to use the M-16, but decide to use the .50 Cal, and the threat is in a group of people, his body sure as shit isn't going to stop a .50 cal round, so I would say that:
1. Dude shooting at you is a legitimate military target
2. The round from a .50 cal is going to continue past said target into the crowd.
3. The .50 cal is not a proportional use of force.
We've war gamed this ad-naseum with the JAGs before every deployment. Yes, you can use a .50 cal, but if something is more proportional - you are supposed to use that.
Ill remember that whenever I end up being shot at by an enemy intermixed among civilians from an area designated as a non-combat zone (church, hospital, mosque). Maybe we could put a placard on all the .50 Cals. Something along the lines of "Do not use if enemy is only shooting at you with small arms from the window of a mosque."
F Hadji, and Tojo, and Charlie, and Ivan, and for that matter anybody else that points a gun/rocket/missile/mortor in my direction.
I guess I didn't word it correctly. What my point is that you're supposed to use proportional force, so if you've got small arms available to respond to a small arm threat that's what you're supposed to use. If you've only got a .50 cal available to respond to a small arms threat, than that's what you're supposed to use. If you make a conscious decision NOT to engage with small arms when they're available to you, and engage with something that's not proportional for whatever reason, then you're violating the laws of war.
Here's what I'm trying to allude to:
1. You're dismounted from your gun truck, carrying your M-16 and M-9. You start taking small arms fire from a group of mixed civilians/insurgents and take cover behind your HMMWV. You look up at the turret gunner and say "take him out." Legal.
2. You're dismounted from your gun truck, carrying your M-16 and M-9. You start taking small arms fire from a group of mixed civilians/insurgents and take cover behind your HMMWV. You look up at the turret and notice that no-one's manning it. You put down the M-16 thinking "I'm gonna waste this motherfucker", get in the truck and climb into the turret and open fire with the .50 cal. Murky, but possibly not legal (you already had something proportional to address the threat with).
3. You're dismounted from your non-gun truck, running a CAS mission with some skids. You start taking fire from small arms in the open - fastest response is to bring the skids in on a 6 line and let loose with their 20mm. Legal.
EDIT: I'm not saying that using any caliber of weapon is legal or illegal. I'm merely saying that proportionality is misunderstood by a lot of people, and that's what could lead to the myth that the .50 cal is not legal for use against personnel.