Megatron fears no one and nothing. Why would he reformat into a disguiseable form if he has already obtained the most powerful, fastest form already (the interplanetary jet mode). He doesn't need to hide, nor would he want to. Any other mode, even based on something human related, would cost him his advantage of the most powerful mode on Cybertron.
Even Gen1 Megatron didn't choose to be a gun, the Ark did it to him while he was offline. Given the choice, I doubt he would have picked it as an alt mode by choice. He would have gone for the biggest most powerful military weapon possible.

It's nothing against the Gen1 gun mode, just an objective comparisson of whether the most powerful Decepticon would want to be a human hand gun or an interplanetary jet fighter.