As Sharky pointed out, it's definitely Grimlock's missile:
http://www.tfu.info/1985/Autobot/Grimlock/grimlock.htm
Just be aware that there are two versions, stumpy missile (like in the link) and long-pointy missile like you have.

It'd say it's probably a gun for a KO Blitzwing, but the only KO I can find is this oversized one. There was a white Diaclone version, but the source is unable to say if it is legit or a KO of the Diaclone toy.