Energy isn't what moves stuff inertia is...
It takes more energy to accelerate a smaller mass to an equal momentum than it does a larger mass...
These two statements directly contradict each other.
Energy is clearly what moves things, in fact, energy is essentially force applied over a distance. In other words, kinetic energy is the measure of how much force it takes to get a given amount of mass moving at a specific velocity over a given amount of acceleration distance, or, conversely, how much force will be required to stop a given amount of mass moving at a specific velocity, over a given amount of deceleration distance.
Inertia is the property of a moving object to remain in motion and to resist changing direction and the property of a non-moving object to remain at rest. Energy is what is expended to create movement and therefore momentum. It is also what is expended to eliminate movement. The force a projectile expends on a target medium in the process of being decelerated over a given distance is the energy of the object times the distance over which the deceleration took place.
...energy is what... stretches stuff...
It is what stretches/deforms things. Which is precisely why it affects expansion of the bullet since deformation and expansion are the same thing.
It takes more energy to accelerate a smaller mass to an equal momentum than it does a larger mass hence the reason a 124 gr 9mm will use more powder than a 147 and will require more pressure to make equal momentum 124+p will get close to the momentum of 147 std pressure rounds they'll also make more noise, so guess what happens as they slow down, that's right they make more pressure more heat and more noise but the momentum remains the same.
It is true that it takes more energy to get a light object moving at a given momentum than it takes to get a heavier object to that same momentum, but your attempt to extrapolate that fact into a practical application of ballistics falls short.
For one thing, it's not just that more heat and noise is generated, it also means that
if they both stop in the same distance, more force will be expended on the target medium by a light object moving at a given momentum than will be expended on the target medium by a heavier object moving at that same momentum. That means that assuming both objects decelerate to a stop in the same distance, at the same momentum, the lighter object (the one with more energy) has the potential to apply more force to the target medium and therefore do more damage.
Real simple if energy is what makes penatration do this simple experiment take a 16 penny nail and try to drive it into a 4 X 4 post with a jewelers hammer, you'll find that you'll sweat like a bandit and make a ton of noise and you'll probably even make the head of the nail hot now try the same thing with a framing hammer.
This is a poor analogy and only makes sense to people who haven't actually run the numbers.
It's typically used to imply that the light hammer has more energy and yet is less effective at driving nails.
The fact is that both momentum AND energy are typically lower with a small, light hammer. One would need to get a jeweler's hammer's 4 oz head moving at two and a half times the speed of a 25 oz framing hammer's head
just to get the energies equal.
That would require an angular swing velocity around three times faster for the jeweler's hammer given the shorter handles on small, light hammers. That's just to get the energies equal--an even faster swing would be required to get
more energy in the light hammer head. While one can swing a light hammer somewhat faster than a heavy one, increasing swing speed by 3 times or more isn't likely at all. In other words, the example is a great way to show that with reduced momentum AND reduced energy it's harder to drive nails.
That said,
if one actually COULD get the head of the light hammer moving fast enough to exceed the energy of the heavier one, basic physics tells us it would require more force to stop the lighter hammer head (the one with more energy) in a given amount of distance. That's the same thing as saying it would be more effective at driving nails than the one with less energy.
So why aren't hammers super light so you can swing them super fast? There's a practical limit on the maximum velocity one can swing a hammer accurately, and given that practical limit, if one wants to increase the energy of the hammer, the only effective way to do it, after a point, is to increase the mass of the hammer head.