reference for the sake of an brown townogy, apples and oranges is what I wanted: loud vs quiet bombers

That last image is a newer concept by german military for a tank cannon suppressor.
The AC130 is not, has not, and will never be a bomber. Again, you are comparing aircraft designed for two distinctly different roles.
That last image has it loving bolted to the ground. are you blind? In the time it takes to get that thing set up, the enemy will have laughed themselves to death.