First of all, I apologize that it took me so long to reply, and that my reply is so long.
Rules of engagement vary with the specific mission, the unit, the combat theater, and even the year. However, the concept of PID (positive identification of threat) is always crucial. PID is the sine qua non of any ROE.
Double-tap is against ROE, and it always will be, because a "double-tap" consists of neutralizing a threat and then shooting the target again for 'good measure' even when it is no longer a threat. If it's not a threat, you're not allowed to shoot it, even if it WAS a threat earlier. If it's no longer a threat, then you don't have PID. If you don't have PID, you mustn't shoot it, even if ordered to, unless you want to get caught under a pissing contest between your Chain of Command and the ROE of your theater.
Double-tap is not to be confused with a controlled pair. Example: Room-clearing team enters the room. Target is acquired. Target is shot twice - bang, bang - and target goes down. That's a controlled pair. We use controlled pairs because the M4, with its shorter barrel (4" shorter than the M16) and collapsible buttstock, has a tendency to ice-pick the target, rather than giving the tumbling we need in order to make a nice hole. However, two holes in close proximity to one another can really mess up your day. Plus there are those blended-metal rounds that we're not allowed to use anymore. :( But I digress.
Example of double-tap: Room-clearing team enters the room. Target is acquired. Target is shot (controlled pair, whatever, doesn't matter). Target goes down. Target is no longer a threat; incapacitation, surrender, death, doesn't really matter. Target is not a threat AND YET some bozo shoots the target a second time because that's what people do in the movies. BAD.
Whether the foe is wounded or not is irrelevant. The question is, do you (the good guy) still have PID (positive identification of a threat/target)? If the guy is no longer a threat, he's not a valid target. It gets more complicated when you're talking about traffic control points, vehicles, etc. but here we're talking primarily about a bunch of guys who are walking down the road, minding their own business, with their weapons (if that's what they are) slung, NOT in their hands. They weren't a threat to begin with. Therefore, the gunner didn't have PID. Therefore, he shouldn't have even asked for permission to fire, because he didn't have PID. His Higher gave him permission to engage (G-d knows why), and from that point on, it was the responsibility of the gunner to kill the targets, period. He had permission (which he shouldn't have, but whatever); from that point on, KILL THEM. Don't half-ass the job and then come back to finish the job when they don't pose any kind of threat.
The worst thing you can do is engage a non-threat, half-ass the job, engage a non-threat AGAIN, and finally engage the non-threat a third time while someone is ferrying the injured to hospital. I know it didn't have a red cross on the side but it walked, talked, quacked like an ambulance. The gunner knew exactly what was going on -- the injured were being taken to get medical attention -- and he engaged the vehicle anyway.
Engaging a vehicle with 30mm cannon fire is fine: 30mm is anti-materiel, and a vehicle counts as materiel. Engaging a group of men with 30mm cannon fire because they MIGHT have weapons slung across their shoulders? I'm not sure whose bright idea that was.
