It is clearly not with him for two shots in the trailer. So do you guys think He will acquire it late in the film? Or does it transform out of his arm, or maybe it's like his G1 version where it can be detached and always stays un-transformed? Also a little side note the concept art and the shot of it in the trailer appear different.

Another interesting thing to note is the very top of the cannon appears to have the same design as Prime's shield gun. Could Megatron's cannon also possibly be a shield?

 

I suspect he can just transform it out of nowhere, considering the cheat transformations of AOE. Not surprised the concept art is different from what appears in the film. That happens all the time. I doubt the cannon is a shield. Any similarities to Prime's shield/gun are likely to reinforce that they're both from the same group and the same era.

 

they're made out of Transformium!

 

I think he is referring to the likes of Bumblebee's cannon. Which just kinda spawns off his arm in the film.