You will note that there is no mention of arrow weight as being a factor in the arrow's flight, once it leaves the bow. This is also the case in rifle bullet ballistics. If a 100 grain 6MM bullet has the same ballistic coefficient as a 200 grain .30 caliber bullet, and they have the same muzzle velocity; they will fly the same out to infinity. Central principle of ballistics. Time of flight is what matters, and that is controlled by the ballistic coeffienct of the projectile as well as the initial muzzle velocity.
True, except for the fact that you are comparing apples and oranges. Same ballistic coefficient, yes. But you have to reduce the size of bullet to retain that same coefficient. Change that 100 gn bullet into a .30 cal like the 200 gn, and you've changed the sectional density dramatically, which also drastically changes the ballistic coefficient.
Fortunately, our arrows are hollow. That gives us some real opportunities to customize our arrows according to our needs. We can change the sectional density of our arrows to improve their ballistic coefficient by adding weights.
We want to improve long range trajectory, or we need better energy retention for better penetration on larger or tougher game, or to shove a broadhead with a large cutting diameter through? No problem.
So, we slide a weight tube inside the shaft, or use brass or stainless steel inserts, or maybe bump up to a 200 gn point instead of a 100 gn point. Or maybe we do ALL those things. And there it is. We've increased the arrow's sectional density and improved it's ballistic coefficient without altering the arrow's diameter or profile at all.