One thumb short,
I also like Powder keg's salute gun, it's a clean, simple tube, and that type of breech is a tried and true design.
I don't have an RBG cannon, but I've handled a few, and they impressed me as quality products.
As for my opinions on choices of proper materials, and the wall thickness needed to ensure a safe cannon, my stock (but truthful) answer is that I'm no metalurgical engineer. Just in considering recoil and appearance, an 8-inch long barrel for a 10-gauge gun sounds a little short to me.