You're going about this backwards. Pick the cartridge 1st, then select the bullet weight that does what you want it to do. In 223 you can get bullets from 45 gr to 90 gr. The lighter bullets leave the muzzle faster and often do more damage to something they hit. They also tend to shoot flatter, at least out to moderate ranges. But there are downsides. While they may do a lot of damage, they often don't give adequate penetration to do that damage to vital organs. And while they are faster and shoot flatter at 1st, they also slow down faster at longish ranges.
A heavier bullet in any given caliber is going to be longer in relation to diameter. It will penetrate deeper when it hits at any range, and is usually more aerodynamic meaning it retains velocity better as ranges increase. While a 75 gr bullet starts out slower than a 55 gr bullet, it will impact at faster speeds at some point down range. It probably will never actually shoot flatter, but will remain supersonic farther down range.
As long as bullets remain faster than the speed of sound they remain stable in flight and accurate. Once speed slows below the speed of sound they become unstable and accuracy is gone. While the less aerodynamic 55 gr bullet shoots flatter, it will also become unstable at closer ranges compared to a 75 gr bullet. There are ways to compensate for the slightly more arched trajectory.
But you reach a point of diminishing returns. In my opinion, for most people around 70-77 gr is about as heavy as a 223 is capable of shooting effectively. If you need a heavier bullet, you need a larger caliber in my opinion. Obviously some disagree because they make 90 gr 223 bullets.
Then there is barrel twist. The rifling in most early 223 rifles was 1:12" designed around light 55 gr bullets. The longer a bullet is, the faster it must be twisted to become stable in flight. Most 223's today are 1:7, 1:8, or 1:9. With an 8 twist more common. Those will usually shoot bullets 50-75 gr well enough. If you're going to shoot 90 gr bullets you need a 1:7
