Let me clarify. You have stated here and above that this might bag some AQ, but we have no guarantees of that. It seems to me that the generation of at least some small amount of good will is inevitable, if perhaps short termed. Thus, I was stating that the guaranteed benefit was the primary one and the potential benefit was secondary. Either way this is beside my point.
I disagree on a few issues here. First I think some weapons will be used against us almost immediately. From what I have seen there is little in the way preventing them from falling into the hands of those who would do us harm, and some of those people would, I guess, relish the chance to use our own weapons against us.
I agree with you that it is not very likely for them to be found on insurgents who just killed some Americans, at least for a while. Firstly, you are right that they will be a very small percentage of the weapons out there, making them a small percentage of the weapons used on us. Secondly, one of the insurgents or terrorists with the that weapon must be killed or captured. Thirdly, there must be someone present who will both recognize where the weapon came from and report that fact at large. I would not expect all of these to come together for a while, and even then for it to take a significant coincidence.
I disagree on the political fallout if this does occur though. Imagine a burglar shoots a homeowner during the burglary. That would make the local news. Suppose, however, that the police had recently instituted a controversial program to give guns to the members of one local gang with the idea that they may take out the really dangerous criminals in the area. Now suppose the same murder-during-a-burglary takes place but the police find that it was one of the guns that they handed out that killed the homeowner. In both cases the homeowner is just as dead, and the burglar would probably have found another gun to use from somewhere, but how different do you think the stories would be represented in the press? What do you think the family of the homeowner would say about the police to the press at every chance they got?
I admit the chance of this coming to pass is not huge, but I sure as hell wouldn’t take the risk.