But... He grabbed the zed with his left hand and then punched it with his right. Neither had the gun in it and then it appeared again...Noticed it myself but didn't want to point it out.
Charlie punching the zombie with the gun would have been funny as hell but I like the idea that he dropped the gun off panel, punched the zombie, then picked it up again. That would also fit with Sam's expression; 'your a total clouts but you did punch a zombie in the face, that will do.'