Boresight, two lines that never meet

[Redleg]

You look through the bore and it is centered on a target at 25 yards then you adjust the crosshairs of the scope to the target to boresight the scope. Do the lines intersect?
http://www.ktgunsmith.com/boresight.htm

 

[gunner1982]

It's my understanding that when I bore sight my AR at 25 meters it is zeroed at 300 meters.

Therefore, the bullet leaves the barrel slightly rising (because of the tiny angle the barrel firing the bullet "upward like a cannonball" - think lobbing a softball) and it crosses the line of sight at 25 meters on its way up and the gravitational pull forces the bullet down in a slight arc that leaves the bullet crossing the line of sight again at 300 meters.

Hence a 25 meter zero being center mass at 300 meters.

I am by no means and expert - but I would say yes for an AR but might be different for other weapon systems.

 

[Redleg]

Gunner, thanks for the response. If you zero at 25 meters at 300 meters the round would impact less than 32 inches below your point of aim given no wind. So on a standard military silhouette, you would score a hit.

 

[George P. Burdell]

So what exactly are you trying to ask?

 

[GAgunLAWbooklet]

the answer to the question is obviously "yes" the scope line intersects with the rifle bore line, at 25 yards.
If you're just looking or shooting laser beams, that's the ONLY point of convergence. The lines cross, and the laser will go higher and higher above a target the farther away it is.

With almost any decently-powerful rifle bullet, the bullet moves up from below the barrel line, crosses it once at the first zero (given as 25 yds.), keeps rising to a peak arc somewhere downrange, then falls to eventually cross the bore line again at the second or longer-distance zero.

If I zero my .243 rifle at 25 yards, it's dead-on again at 100. From there it drops, but since it's a fast bullet, it doesn't drop all that much. Only 8.7" of hold-over required at 300 yards.

 

[GAgunLAWbooklet]

With a .22 rifle or 9mm carbine, with a scope centered 1.5" above the bore line, your 25 yard zero will only give you an insignificant (0.6") maximum-high point around 40-50 yards, and by the time it gets out to a 100 yard target the bullet will be 6" low.

 

[Redleg]

So what exactly are you trying to ask?

Hey George, asking for opinions and thoughts. One method boresight deals with lines that run parallel and the other zero deals with a trajectory that slopes upward.

 

[GAgunLAWbooklet]

If the scope is mounted between 1.5 and 2.0 inches above the bore, you can never "ZERO" the gun while having PARALLEL lines. The bullet would hit that same 1.7" (or whatever) below the crosshairs at any close distance where drop is not noticeable, and from there the drop would only get bigger and bigger.

(Of course the bullet's path would not be a straight line, so it can't be parallel to a truly straight line-of-sight through the scope, but it would seem like a straight line out to maybe 50 yards with a high velocity round).

 

[Redleg]

Hey GAgun, this is going to be a great discussion, I was under the same impression, however, if the line of sight and the line of bore run parallel, they would never meet. Not talking bullet path, just the lines themselves, even in the article that I included in the start of the thread, the author talks about an angle that had to be determined from a ballistic program, and that is not true. Ballistics has nothing to do with boresight, it has something to do with trajectories.

 

[Fatal_Bert]

cmshoot these boys desperately need your help.