I'm not trying to criticize your work or anything, I like the mech a lot its just a funny place for a cannon.
Guns of that size produce immense amounts of recoil and weigh a lot, I imagine the first person to ever design a mech like this took that into account and put it under the body to balance the weight.
Now it just seems like common fact that a gun on a mech has to be under the body unless it's complex like a Gundam or whatever