Kerry lied
Sorry, I usually don't like to resort to the l-word, but gun rights are important to me, and there's a lot of misunderstanding out there. Automatic AK-47's were banned in 1987, and they will not become legal again now that the recent "assault weapons" ban has expired.
Moreover, I don't see how he can claim to support the Second Amendment at the same time he's supporting a gun ban. The NRA, by the way, has given the senator an "F" rating for supporting gun rights.