Here's what I have now. Made the gun overall feel a LOT more stubby, fixing a lot of the proportional issues I saw relative to the character's feel. I'm definitely getting heavy influence by a couple SMG designs from WWII, though game wise, it's more familiar with Bioshock Infinite's Machine Gun, of course :P
So, I'm fortunate enough that the shapes and geometry I'm doing through subD moodeling is mostly just realy simple control edge placements with simple extrusions of cylinders and squares. I still feel incredibly raw doing complex shapes/intersections, and this usually happens when I'm well into the attempt and I don't know how to fix it midstream.
In terms of specific examples:
The cooling jacket examples are frequent enough on the internet that I figured out to just grab vertices in a pattern and chamfer them. It was just when I tried adding thickness to the plane through extrusion that the holes wouldn't be perfectly circle, or the hole isn't a clean cylinder. Unless it doesn't have to be?
What I have here was not numericaly clinical, I kinda just set all the inner loops of the circles zeroed to a common location per vertex and then scaling it by hand. It didn't feel precise, though perhaps it's more important it LOOKS the part as oppose to exactly IS (which I think I have now).
Other places, like the receiver and the charge handle housing and ammo housing (both boxes), had unsatisfactory solutions for me. Right now they're split into three different geos, but I know there's a way to make it all one geo starting from the receiver's base mesh, but I . . . I don't know how to do it. Tried solving it with Adding Edge Loops but it's still hard for me to "flow" through the challenge, and I get stuck. And then I just split the geo and get it all "hacked together." It's overall just learning SubD I guess. I'm still really trying hard to find a comprehensive tutorial online that I can jive with that does teach a good portion of SubD challenges, or maybe I should just buy that tutorial from 3Dmotive.