In Transformers, (I assume u mean the G1 series?), Megatron transforms into a Walther-P38 hand-gun w/various attachments that make him seem more like a rifle. When he is in this mode he can be held by fellow decepticons and used as a blaster rifle. His blasts are more concentrated in this form, and therefor more lethal. The new Transformers movie, the first one, Megatron transforms into some kind of Cybertronian aircraft, in the second movie he is more like a tank of some sort.