Well, it IS dumb, because you can make rolls beyond 1d6 and 1d10, you know, like, 1d6+2, 2d10-3, 3d6+2d10-10x34.
And if you are comparing a SW cannon with some howitzer-like cannon, i think you should divide them by laser and gunpowder and not SW and Earthly.
Do you get my point? Do different stats for each one, not just add 1d6 for SW cannons.
EXAMPLE: A 2d6 explosive damage cannon that has a UR of 4 is good for you.