A question about bullet weight trajectory.

[tpcollins]

I was at my in-laws for dinner tonight and my brother-in-law and I were talking about shooting. He said that it took him awhile to understand this, but because a lighter bullet shoots flatter, it will hit lower on a target than a heavier bullet. I told him I didn't realize that the law of physics had changed and couldn't believe this. He said several people at gun shops had told him this. He said it was because the heavy bullet had a higher trajectory and the lighter buller was flatter. I told him if the gun is pointed in the same direction, the heavier bullet should hit lower. A bullet only rises because the barrel's pointed above line of sight.

Then I told him when I was at the range last week, the lighter 385 grain Remington sabots hit 2" high on the target at 100 yards, the 1 1/16 oz Lightfields hit dead center, and the 1 1/4 oz Hastings hit the lowest. This confirms my understanding of weight, fps, and trajectories. Am I correct in my thinking or is there some truth to what he says? Thanks.

[dmazur]

Assuming the barrel is not allowed to rise during firing, you are generally correct.

It isn't bullet weight that causes the more arched trajectory, but ammunition loaded to pressure limits. Heavier bullets have to have a lower muzzle velocity for a similar chamber pressure, and lower velocity generally results in a more arched trajectory.

If you "cheat", you can find long for caliber bullets that are heavier than lighter bullets and have a flatter trajectory at longer ranges, due to their much higher ballistics coefficient (BC), even though their MV is lower than the lighter bullet's MV.

Another "cheat" is considering dwell time in guns that rise significantly on recoil, such as pistols. It is possible for heavier bullets to hit higher than lighter bullets. This isn't trajectories and BC at play here, but the fact that the heavier bullet is in the barrel slightly longer than the lighter bullet. By the time it exits the muzzle, its POI is going to be a little higher than the bullet that exited sooner.

When I get asked questions like this, which isn't often (thankfully), I usually ask how much detail the person is willing to suffer... I understand that in some parts of the country, calculus is considered a form of torture.

[B.L.E.]

A fast bullet has a flatter trajectory than a slow bullet and the lighter bullets go faster in the same gun.
Now if you are talking about 1000 yard shooting, the heavier bullets may actually have a lower time of flight because they tend to keep their velocity instead of slowing down like ping-pong balls and even though they started off slower, they end up faster as they travel through hundreds of yards of air.

Many guns, especially revolvers, will have a higher point of impact with a heavy bullet than a light one. That's because of the muzzle jump while the bullet is still in the barrel. With my .44 magnum revolver, switching from 240 grain to 180 grain bullets means adjusting the rear sight elevation screw about 1.5 turns to get the same point of impact.

[Scorch]

There are several instances where your brother-in-law might be correct. Out of a handgun, faster bullets generally hit lower on a target than slower bullets if you are using the same sight setting. The reason is that as you fire a handgun, the barrel begins to rise almost as soon as the bullet starts to move. A slower bullet will be in the barrel longer than a faster bullet, therefore it leaves the barrel after the barrel has risen farther than it would have with a faster bullet.

With a rifle, there are too many variables to predict where a heavy vs light bullet will impact given a certain sight setting.