Granted everyone has a different preference, but just curious what some of the theories are...

-MG

Sure, but first tell us what CG stands for

WTH is CG?  In order to answer we need to understand the question.  

Quoted: Sure, but first tell us what CG stands for

CG I beleive is Center of Gravity.

Referring to the question, well in my opinion, a weapon w/ a lighter barrel such as my Colt Gov't Carbine has a good "balanced" feel, where when I'm reloading (cheekweld - w/ weapon on target), it isn't muzzle heavy-causing my front sight to drop.  This is an ideal CG.  My Krieger HBAR barreled 16" upper was a tack-driver, but was very front heavy, and much more effort was exerted in keeping the same "weapon on target reloading drill".  Some SEAL's in Panama were teaching us drills where we fine tuned fast reloading, maintaining cheek weld and sight picture, while the other hand was grabbing a mag, re-loading, and sending the bolt home...then continued "busting them"   We were using M16 A2's w/ irons and M4's w/ Aimpoints.  

You hear about "CG" a lot w/ aircraft, where "CG shifts" such as too much aft or forward weight of CG will affect aircraft performance, like having to use a lot more runway before lifting off/rotate.  

So you want theory?

Well, you want the CoG to be far enough forward to where the rifle isn't too whippy.  However, this would change as the overal rifle weight increase do to accesories, stock choices, optics, etc.  A heavier rifle would probably be ok with the CoG further back than a lighter rifle.  Barrel length also plays a part here too.  So not only is where the CoG is located, but also how much rotational intertia the system has.

Is that good enough theory for you?

In other words, it has to swing a certain way and it has to sit a certain way.  More importantly than how it swings is how it STARTS to swing.  Does the rifle want to roll?  Should I move my support hand?

So you can't just move some stuff around and have the rifle handle well.  You can help, but the bottom line is that some combination are just too heavy or too poorly balanced to work *juuussst riiiggghhhtttt*    Said Goldie Locks

*edit* I type(and think) like an idiot when I am really tired.

sorry, I meant center of gravity (CG)

So I guess the thing to do is to try the reloading drill with different accessory configurations, or even throw a few ounces of dead weight on either end to exagerate the effects of a CG too far forward, or rearward...  

I find the rifle handles best for me when it balances right behind the barrel nut.

Quoted: I find the rifle handles best for me when it balances right behind the barrel nut.

I agree with Bartholomew. To me, if it balances in the right around the bbl nut, that means the weight is evenly distributed across the whole rifle.