I was wondering if someone could confirm a question I have.

I faintly remember Megatron using his Alt Mode Parts to create the huge fusion Cannon mode for a brief scene in the cartoon.

Am I wrong?

 

I don't recall ever seeing that in the cartoon, but he did fire other big ol' sci-fi cannons that you may be remembering.

 

I think he had a duel for two seconds with Optimus using combat deck in mtmte.