Cannons are powerful and helmets protect you so what would serve better as a powerful protection than a cannon mounted on a helmet. The cannon shouldn't be too strong and only be able to shoot once before you have to reload so the weapon doesn't get too powerful, because in the end it is still a helmet. I got the Idea from here. Picture of my idea (I'm not a skillful artist )
Lawls Like you fire once then you're dead. Actually it could be cool but you would have to add a neck protection thingy of some sort.
I can see it now. Before you fire, you have to extend a rod from the ground to your head. Genius! Back Blast area CLEAR!