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# Why does point of impact change?

Discussion in 'Caliber Corner' started by ratf51, Sep 13, 2010.

1. ### ratf51

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I have a Glock 22. It shoots to what I have always heard referred to as "point of aim" with 180gr bullets. When I shoot lighter bullets (165gr, 155gr, ocassionally 135gr) point of impact drops. What I want to know is why? What physics concept, what principle of ballistic arc, what shooter's voo-doo is at work here? Is there anyone who can explain this for me, please?

2. ### BOGEMillennium Member

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Mechanics

Weight (force of gravity) decreases as you move away from the earth by distance squared.
Mass and inertia are the same thing.
Constant velocity and zero velocity means the net force is zero and acceleration is zero.
Weight (in newtons) is mass x acceleration (w = mg). Mass is not weight!
Velocity, displacement , momentum, force and acceleration are vectors.
Speed, distance [d], time, and energy (joules) are scalar quantities.
The slope of the velocity-time graph is acceleration.
At zero (0) degrees two vectors have a resultant equal to their sum. At 180 degrees two vectors have a resultant equal to their difference. From the difference to the sum is the total range of possible resultants.
Centripetal force and centripetal acceleration vectors are toward the center of the circle- while the velocity vector is tangent to the circle.
An unbalanced force (object not in equilibrium) must produce acceleration.
The slope of the distance-tine graph is velocity.
The equilibrant force is equal in magnitude but opposite in direction to the resultant vector.
Momentum is conserved in all collision systems.
Magnitude is a term use to state how large a vector quantity is.

http://www.regentsprep.org/regents/physics/101facts/101facts.cfm

3. ### bentbiker

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They way I've had it explained to me . . . Heavier bullets take longer to leave the barrel. With ignition the muzzle starts to flip up. The longer the bullet is in the rising barrel, the higher the POI. Therefore, heavier bullets often have a higher POI.

Last edited: Sep 13, 2010
4. ### ratf51

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Alright! And fact number 101 on the paper is "Physics is fun!" (great reference, though.) Unfortunately it doesn't explain it for me! Can we get some "Mr. Wizard" or "Bill Nye the Science Guy" kinda thing going here? One syllable words, please.

5. ### ratf51

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Thank you, bentbiker (aka Mr. Wizard!). Makes sense. If I want to get point of aim set for the intermediate weight bullets should I get a higher or lower front sight? Right now I just try to compensate by holding over slightly with the lighter bullets.

6. ### wrenrj1

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Watch the History Channel and catch the shows about snipers etc. and how they have to adjust to hit their targets. Simply, bullets go out of a rifle, and are subject to several forces.

7. ### bentbiker

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When adjusting POI with a front sight, you do the opposite of what you want to happen to POI -- raise sight to lower POI. Opposite left and right as well if you have gun with dovetail in the front.

When adjusting POI with rear sight, move it the same way you want to move POI.

8. ### ratf51

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You, sir, are a scholar and a gentleman.

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