Muahah! At least you like 'im more than you used to.
With the gun on his arm you can turn his arm so he's holding the cannon with his other arm. Gives it a really fun shotgun vibe, which is great for playing with, shooting other bots to shreds and blowing shit up.
I usually keep the cannon off like you did in the review though. The little hook bit left on his arm looks to me like a movie-style arm sword thing, so I use it as that. A twisted hook-blade he uses to lift up skulls from the ground to laugh at.
I also love the way the chest is, how it opens during transformation. It's kewl!