> But that's exactly correct, isn't it? The hero used his own gun to kill some agents during the puppy scene and again during the raid. Then the hero switches guns with his partner, so his partner will have the gun that was used to commit the murders.

Sounds plausible, but that's not what was implied by other events. The partner tells the boss to check the trace (or some such technobabble) on the gun which was used to kill the agents in the puppy incident. The boss does, and says that it is the partner's gun. not the gun he's CARRYING, mind you, but HIS gun. Then the partner checks his gun, and sees that he has the hero's gun (it has the hero's name on it), and says that the hero must have switched them, because he has the hero's gun -- to which the hero replies, "Of course you have my guns. You took them from me when you arrested me" which is enough to satisfy everyone of the partner's guilt (sort of).

> Had the hero killed agents *after* the gun switch, then obviously the gun used would not have been the one his partner was carrying.

Maybe I just misunderstood that whole scene. I think I need to watch that movie again.