Boresight, two lines that never meet

[Redleg]

There are three lines in boresighting, the gun-target line or line of sight, the line of bore and line of displacement. Look at the geometry. Most of the time folks talk about the line of sight and the line of bore meeting, however, in reality, the lines are parallel and there is a third line that meets with the line of bore, the line of displacement, this line is the adjustment that is made to the crosshairs to intersect with line of bore at the target. This is boresight, not zero so no ballistics discussed in this example. The line of displacement intersects with the line of the bore at the target not the line of sight.

 

[CBeck]

Throw on a 20moa base and the line of sight and the bore aren't parallel.

 

[BDR1]

Bullets don't travel above the barrel line. If you were to shoot any rifle with the barrel perfectly level to earth the bullets begins to drop the second it leaves the barrel. In order to hit a target at any distance you have to point the barrel upwards, hence the trajectory and two different zero's.

This is what I was taught....bullets drop as soon as it leaves barrel then starts a slight upward arc,then after a distance it starts to to drop....hence zero at 25 ....am I wrong

 

[Redleg]

Wouldn't the lines be parallel and 20 moa more difference in scope height?

 

[benjamin]

maybe this will give some thought on bullet drop. If barrel is level with plane and you fire it and at the same time drop a bullet, both will hit the ground at the same time, hence substitute a 22 of 30-06 and they will hit the ground at the same time.

 

[Redleg]

This is what I was taught....bullets drop as soon as it leaves barrel then starts a slight upward arc,then after a distance it starts to to drop....hence zero at 25 ....am I wrong

BDR1, bullet drops as soon as it leaves the barrel and continues to drop (gravity, air and all other variables) unless you angle the barrel to attain further distance, once you upward angle the barrel you can identify two points along the trajectory let say 25 yards and 200 yards with the max rise being about 4 inches. This is exterior ballistics and the short answer is you are correct.

 

[Russell0331]

I look through the bore to initially boresight. If the scope is going to be coming off the rifle for storage, then I’ll use a laser boresight to register the scope.

 

[CBeck]

Wouldn't the lines be parallel and 20 moa more difference in scope height?

Bases with elevation built into them change the angle of the scope in relation to the bore. Jack the scope up in the rear.

 

[Fatal_Bert]

This is what I was taught....bullets drop as soon as it leaves barrel then starts a slight upward arc,then after a distance it starts to to drop....hence zero at 25 ....am I wrong

Bullets don't go up and down on their own, bullets have no lift. The Arc is because you're literally pointing the gun in the air in order to hit a target at a distance. Since gravity immediately takes an effect on the bullet in order to hit a target that's on the same level ground as you 100 yards away you have to first send the bullet up.

This is what creates the rise, you're pointing the barrel upwards, while the bullets trajectory travels up then down your sight line is a straight line that crosses that trajectory twice. There is no upward arc if your barrel is completely level or pointing down like when shooting from a deer stand.

This diagram does a great job of explaining why the two zero's. Notice the bullet never rises above the bore line. I could be reading some of these responses wrong but it seems some are under the impression that it does.

 

[lostchild]

1: Invalid poll, no tacos choice.
2: just zero the thing and be happy.
3: ODT isn't a Tech math class, you're making us dummies' heads hurt.