It's that simple. If it isn't working, I don't know what to tell you. Vert spacing along the loop doesn't matter; as long as it's a proper edge loop it should turn it into a planar circle:
the shift-click is, IIRC, the loop/ring selection. so if it doesn't work, it means you're trying to select edges that aren't in the same loop/ring. You need quads to pull this off.
My first recommendation is removing any lines that loop around and doesn't contribute to any of the box's shape. I see about three loops you can delete right away.
Yeah, Ctrl+backspace is default in max. I found it's pretty nice to bind "remove loop" to a key. Just select one edge and hit the key and it removes the loop plus the vertices.
After some experimenting. This edge loop method works best because putting an extra edge loop next to main edges reduces smoothing errors. This is working out fine now.
You ought to be able to insert a control loop just in the area of the extrusions and then use an ngon or triangle to terminate that loop. A few more segments will help but it isn't necessary to use an extreme number.
panel loops might be part of what you are after. It has polish and bevel features. It's pretty awesome tbh. https://pixologic.com/zclassroom/lesson/panel-loops-options-part-1
Looking much better with the loop in there, I personally feel the lifting of the minigun after the loop seems a touch linear it moves pretty much straight up. Just my personal thought on it :)
Yep add some up n down loops from the corners of the middle indent. For hipoly models, correct shading is way more important than a couple extra edge loops.
I've been experimenting with the Panel Loops and Group Loops features recently, and thought I'd try them out with creating seams and hemlines for clothes. Study 26 - Cloth Study Boxers