The original Megatron was a bad-ass dude who wanted the Decepticons to get some energon cubes. He could transform into a Walters P-38 Pistol that the other Decepticons could use as a weapon.

Several years later Transformers came out with Beast Wars.
Megatron II is the leader of the Predacons in the Beast Wars (aka Beasties) television series.
He transforms from a humanoid robot to a tyrannosaurus rex. Not really that good of an idea because Grimlock was already a character.
In the upcoming live-action film, Megatron will allegedly appear as an alien jet.
The apparent explanation for the departure from his gun form is that director Michael Bay does not want to include any sort of size changing in the transformations.
Another explanation could be that Michael Bay is a piece of shit.

Now Hasbro is re-releasing the G1 toys to support the movie
All I can say is that toys look OK except that Megatron looks like a god damn supersoaker. WTF!?

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