Same PF why would a lighter buller shoot higher?

[ProGunGuy]

so i've been playing with 158 gr moly out of my Glock 34. I usually run 147's. the 158's a re going about 850 and the 147s about 900.

Most of the times i have used heavier bullets they would print higher. the opposite is true with the 158's. they shoot 2-3 " lower than the 147's at 15 yards.

I've been trying to figure out how come. i would like to use them interchangably without having to change my sights.

Any have any opinion on why the 158's hit lower?

[dab]

Could it be that since the 158's have a slower velocity, they are in the air longer before hitting 15 yards. Since they are in the air longer, they have more time to fall than the 147's. I don't know if 50 fps would make a 2-3" difference at 15 yards, though.

[mcracco]

Could it be that since the 158's have a slower velocity, they are in the air longer before hitting 15 yards. Since they are in the air longer, they have more time to fall than the 147's. I don't know if 50 fps would make a 2-3" difference at 15 yards, though.

Actually, not the case. Gravity acts the same on any projectile regardless of velocity. A projectile leaving the muzzle at 200 fps will hit the dirt at the same time as a projectile traveling at 2,000 fps given the trajectory is the same.

[SlowShooter]

Could it be that since the 158's have a slower velocity, they are in the air longer before hitting 15 yards. Since they are in the air longer, they have more time to fall than the 147's. I don't know if 50 fps would make a 2-3" difference at 15 yards, though.

Actually, not the case. Gravity acts the same on any projectile regardless of velocity. A projectile leaving the muzzle at 200 fps will hit the dirt at the same time as a projectile traveling at 2,000 fps given the trajectory is the same.

I think Mcracco and Dab are talking about different situations:

Assuming all shots with barrel exactly horizontal, along an infinite ground plane, then: All bullet weights will hit the ground at the same "time" (they all fall at the same rate) but 'Trajectory' is different because a faster bullet travels farther (horizontally) in the same amount of time before it hits the ground.

I think Dab might have been thinking of the case where the muzzle is horizontal and shoots at a vertical paper target. If shooting at a vertical piece of paper, then yes the faster bullet gets to the paper before the slower bullet, thus the slower bullet "spends more time in flight" (more time to fall) before it strikes the paper.

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FWIW, I have seen this effect also (different POI with different weights) and I think it has very little/nothing to do with gravity or time-of-flight; I suspect but cannot prove that it is actually an effect of the mechanics of the pistol (barrel unlocking tilting? Or recoil forces?) because really I don't see how 50-100 FPS difference at a close-range pistol target amounts to enough of difference when gravity/falling effect is concerned.

If you have a lighter bullet traveling at 1000 FPS and a heavier bullet at 900 FPS, at a vertical paper target 50 feet away then the time-of-flight difference is only 0.0055 of a second (5.5 milliseconds).

Edited March 18, 2013 by SlowShooter