Paralax?
This may be a simple question to some of you more knowledgeable guys, but I suck at physics and need an explanation. I recently purchased a flat top upper for my AR because I didn't like having the scope on the carry handle 7 inches above the barrel. I installed the scope on the new upper and fired a couple shots at 25 yards to make sure I was on paper. I was only an inch or so low so I corrected the point of impact and moved the target out to 50 yards. At 50 yards I was grouping about 5 inches high. I lowered it and moved it to 75 yards. Now it's three inches high. Another correction and moved it to 100 only to be 2 inches high. Another adjustment put it steadily in the center. I ran out of ammo and didn't get to see what happens if I move it back to 50 yards. It groups great at a set distance Any ideas what's going on?
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Look up parabolic ballistics. 0 -
The bullet needs to climb up to the line of sight of the crosshairs. It continues to keep rising until gravity pulls it back down to the line of sight. From then on the bullet will strike below the point of aim. At those to specific distances the bullet will strike at the point of aim.
https://www.hornady.com/team-hornady/ballistic-calculators/ballistic-resources/external-ballistics
Height above the bore, initial velocity and the ballistic coefficient of the bullet are factors in your trajectory.
Spend some time playing with a ballistic program to see what each factor does to the trajectory.
https://www.hornady.com/team-hornady/ballistic-calculators/#!/0 -
Thanks for the input, I'll do some more research. My reloading manual lists the mid range trajectory at about 1 inch at 100 yards. I had to lower it almost 7 inches. Something just seemed strange. 0 -
The scenario described does not involve "parallax", it's simply the rise of the bullet from bore line to scope axis. 0 -
quote:Originally posted by Mobuck
The scenario described does not involve "parallax", it's simply the rise of the bullet from bore line to scope axis.
Yes, I had to sight my AR to 1-1/2" low at 25 yards to be 2" high at 100 yards.0 -
the height of the scope mount comes into play more than anything else the bore of the barrel is solid and straight but you start out below the bore the flight of the bullet is an ARC starts out below the bore line then goes above bore line then goes same as as bore line and then goes below bore line 0 -
One thing to remember that's different about ARs. With a typical bolt action gun the centerline of the scope is generally as close to the center line of the barrel as the mounts will permit. In other words the scope is close to the barrel. Because of the design of the AR platform the hight of the scope above the bore is greater. This causes exactly what you experienced when the gun is sighted in. 0 -
Hello,
Let's clarify something first. The bullet does not rise after it leaves the muzzle if the barrel is perfectly horizontal. There is some REAL good information in this link. http://www.rifleshootermag.com/shooting-tips/shooting_tips_ballistics_0303/
Michael0 -
quote:Originally posted by Mod1892
Hello,
Let's clarify something first. The bullet does not rise after it leaves the muzzle if the barrel is perfectly horizontal. There is some REAL good information in this link. http://www.rifleshootermag.com/shooting-tips/shooting_tips_ballistics_0303/
Michael
Not sure if you knew this but nobody fires a rifle with the barrel perfectly horizontal. So in practical terms the bullet is rising to the scope axis.0 -
It's relative. 0
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