To be fair...
One has to be careful evaluating photos. When dozens of shots are taken in rapid sequence, it's almost impossible not to have one of them that makes you look stupid.
In the case of this photo, one caption I saw said that Kerry was reacting to missing a clay pigeon, implying that he had fired the gun only seconds before, and was lowering it after the shot with a "dang!" expression on his face.
If you did not know this, it looks like he is just goofing off and waving a gun around.