Beware, I'm just a humble keyboard physicist
So, I've been doing some reading since my last post and from what I can tell, the bullet becomes more stable not because the center of mass is moving to the rear, but in spite of it. The bullet becomes more stable because you are moving a larger percentage of mass closer to the center of mass (by removing mass that is furthest out). This means that less mass of the bullet as a whole is trying to spin the wrong way, requiring less good spin to stabilize the bullet. Look up "transverse moment of inertia." But you are still moving the center of mass further and further behind the center of drag as you continue to remove mass from the nose and eventually stability will peak and then begin to drop.
So to answer the question:
Why does the same feature (center of mass being aft) stabilize a bullet in air and destabilize it in water?
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It doesn't. The center of mass being aft of the center of drag would cause the bullet to tumble right out of the muzzle if not for the spin imparted to it by the rifling. It is a destabilizing factor. Like Troy explained, it's just much easier to overcome in the air.
One thing that should be noted when removing mass the from the nose... is that you are removing mass!
So unless you're creating a feature like a ballistic tip or a hollow point, why do it? A shorter bullet with the same weight and profile will be even more stable and have a better B.C. Looking at a 77gr Nosler HPBT, it seems to have about 0.2" of air in the nose. If you were to try to squeeze that out of there you'd probably only shave a couple thousandths off the bullet's OAL, but hey, it'd still be more stable [and you could pack just a little more powder behind it, if that's your sort of thing].
Well, maybe it's stable enough. Perhaps Nosler got it just right for a certain twist and a certain velocity they were using. Can something be too stable? How stable should a bullet be? Just enough and no more. [Shamelessly stolen quote.] Like the ammo-oracle describes, the bullet can go nose up or even burst. (Read
here for accurate descriptions of what I'm talking about.) Going nose up creates drag, and loss in velocity. As the bullet spins, imbalances in its construction will make it wobble. More spin, more wobble, less accuracy. [Wobble is a highly scientific term.
] So, why not reduce the spin and shrink the bullet? That would be better all around but who wants to buy a 1-8.2" twist barrel just to make one stinkin bullet fly better? Or maybe the bullet's construction is just a result of the machines being set up "good enough." Or perhaps a built-in margin-of-error. Whatever. Two hundredths of air or two thousandths of length or two grains of lead isn't going to mean crap to 99% of the people who shoot them anyway.
I'll stop now because it seems I've lost all notion of direction in this post. But I do believe the first two paragraphs explain a lot about what was originally asked. I've tried to be as accurate as possible but claim no lack of errors in my post. Corrections are welcome. I'll go crawl back in my hole now.