Actually, I tried it and the Galvatron cannon from the tank does clip on to the arm tabs but it doesn't hold on as well as the Megatron cannon. It also hangs a little low for my liking. The tip of the barrel almost touches his feet. Also, there is no way to attach it in alt mode without a bit of modding. Hmm, I might look into adding that cannon but I will get the robot finished first. The Megatron cannon will be getting a new paint job in Galvatron colors btw. I also took a look at the "ruined" Megatron head. Surprisingly, the screw holds the head perfectly in place. God knows why Hasbro glued it together. However, this now gives me a spare "battle damaged" Megatron head. I look forward to using it.