It's not nearly as technical as we want it to be. I’ll try to use real life terms to communicate the truth about beliefs surrounding these two topics of discussion. By the way, the points mentioned here will hopefully simplify the process of deciding what gun to get your youngster for their first deer hunt (God bless you by the way).
Here's the truth about brush-busting bullets and knockdown power: Every bullet deflects in brush. Bullets kill by causing tissue damage. Bullets don't knock down deer.
Like most hunters, I took a fair number of deer with a .30-30. One positive aspect of this round was that it was supposed to be a good brush buster. However, a bullet from a .30-30 will deflect in brush just like a bullet from a .243 will. What makes a .30-30 a good brush gun is it's chambered in short, handy rifles. I’ve personally witnessed bullets fired from a .30-30 and other calibers; some with heavier bullets, some with lighter bullets, some going faster, others going slower, miss the mark by a wide margin after encountering high grass or a twig. Bottom line is if you have a spinning, lead projectile flying through the air at a couple thousand feet per second and it hits something, it is likely to veer off coarse.
Bullet weight is given in grains. There is 7000 grains in a pound. A 150 or 170 grain bullet is heavier than a 100 grain bullet from a .243, but neither is an extremely heavy piece of weight. Thus, bullets deflect.
What about knockdown power? Here is where I refer to the laws of physics. For every action, there is an opposite and equal reaction. This means that the amount of momentum a bullet delivers to the target can be no greater than the amount of push, or kick, generated by firing the round. This momentum is what you feel when the gun recoils into your shoulder. Think of it this way, you can't create energy. You only change its state. Due to trauma within a deer's nervous system, it reacts in a way that may make it appear as if it was knocked down. When you see the guy get blown across a room in the movies, remember it's a movie. Don't get me wrong; there is an impact, but not nearly enough to lift a 150-200lb deer off its feet.
So what is a good rifle for deer, especially for young hunters? It's whatever rifle is chambered in a legal deer hunting round that can be shot accurately. An eight pound sledgehammer won't put in a finishing nail if you can't hit the nails head. Likewise, a cannon won't kill a deer if you shoot over it's back. Once you have a good shooting gun, don't fret too much over the bullet. Years of hunting has shown me that a good, expanding soft point bullet designed for deer will put a hole around an inch through the tissue it hits. Folks, a one-inch hole in anything's plumbing is going to bring it down.