I was just wondering how it works...


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Snipy
August 6, 2007, 08:49 PM
Hello and sorry for the bad title... I don't know what fits well my question... :D
I was just wondering why if the sight is distant aprox 1 inch from the barrel, if I shoot, the bullet goes where I aim and It doesn't go 1 inch down...
Hope I explained well my question...
Bye!

Alex.

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dmazur
August 6, 2007, 09:17 PM
Bullet follows a curved trajectory, generally crossing the line of sight twice. Before the first point of crossing, the point of aim is above the point of impact. Between the first and second points of crossing, the point of aim is below the point of impact. Finally, after the second point of crossing, the point of aim is above the point of impact (until the bullet hits the ground...)

If you adjust the sight to be absolutely parallel to the bore, the POA is always above the POI. (Rifle always shoots low)

By choosing a "sighting in" distance, you are adjusting the sight to be zeroed at some point. Say 100 yds. At this distance POI is the same as POA. What is interesting is that POA is also the same as POI at some closer distance, say 25 yds, but this distance is too close to be considered useful.

Snipy
August 6, 2007, 09:26 PM
Thanks for the explanation :) .

MechAg94
August 7, 2007, 01:09 PM
Yeah, that fore knowledge comes in handy the first time you shoot and AR15 with irons at 25 yards. You have to remember it is supposed to shoot there.

LHB1
August 7, 2007, 03:26 PM
Because the sights are designed such that they point slightly downward in comparison to the barrel bore line. Thus the bullets, following the bore line and then being pulled downward by gravity, cross the line of sight twice as described above. The second crossing point is usually described as the distance at which the sights are zeroed.

Good shooting and be safe.
LB

benEzra
August 7, 2007, 09:05 PM
Here's the trajectory of a bullet fired from my SAR-1 (civilian AK lookalike). Sight height is 3.8 inches above the bore to the center of the sight's lens.

http://www.commongroundcommonsense.org/forums/uploads/1168567538/med_gallery_260_23_20379.jpg

http://www.eskimo.com/~jbm/cgi-bin/jbmtraj.cgi

Trajectory Output
Input Data
Ballistic Coefficient: 0.285 G1 Bullet Weight: 123.00 gr
Diameter: 0.311 in
Muzzle Velocity: 2350.0 ft/s Chronograph Distance: 10.0 ft
Sight Height: 3.80 in Sight Offset: 0.00 in
Zero Height: 0.00 in Zero Offset: 0.00 in
Azimuth: 0.00 MOA Elevation: 0.00 MOA
LOS Angle: 0.0 deg Cant Angle: 0.0 deg
Wind Speed: 0.0 mph Wind Angle: 90.0 deg
Target Speed: 0.0 mph Target Angle: 90.0 deg
Temperature: 75.0 F Pressure: 29.92 in Hg
Relative Humidity: 70.0 % Altitude: 200 ft
Vital Zone Radius: 5.0 in Std. Atmosphere at Altitude: No
Corrected Pressure: Yes Target Relative Drops: Yes
Zero at Max. Point Blank Range: No
Calculated Parameters
Elevation: 8.91 MOA Azimuth: 0.00 MOA
Atmospheric Density: 0.07309 lbs/ft Speed of Sound: 1133.6 ft/s
Maximum PBR: 287 yds Maximum PBR Zero: 248 yds
Range at Max Height: 148 yds Energy at PBR: 713.9 ft•lbs
Sectional Density: 0.182 lbs/in

Calculated Table

Range Drop
(yds) (in)
0 -3.8
10 -2.9
20 -2.1
30 -1.3
40 -0.6
50 0.1
60 0.6
70 1.1
80 1.5
90 1.9
100 2.1
110 2.3
120 2.4
130 2.4
140 2.4
150 2.2
160 2.0
170 1.6
180 1.2
190 0.6
200 -0.0
210 -0.7
220 -1.6
230 -2.6
240 -3.6
250 -4.8
260 -6.2
270 -7.6
280 -9.2
290 -10.8
300 -12.7
...
400 -39.2
...
500 -84.0

Note that the bullet starts out 3.8 inches below the line of sight, but the barrel is pointed up very slightly (to launch the bullet slightly upward) when the sights are aimed at the target.

The bullet is traveling upward toward the line of sight to start with, crosses the line of sight around 50 yards, reaches a peak of 2.4 inches above the line of sight around 140 yards, then starts dropping. It crosses the line of sight again at 200 yards, and is more than a foot low by the time it gets to 300 yards, over 3 feet low at 400 yards, and seven feet low at 500 yards.

That trajectory is what happens when the rifle is zeroed at 200 yards (i.e., I adjust the optic so that the bullet crosses the line of sight at 200 yards). Conveniently, zeroing at 200 with this sight height means the bullet is only a tenth of an inch high at 50 yards (only a third the diameter of the bullet), meaning if I sight in the rifle at 50 yards, it should be nearly spot-on at 200 as well.

pdowg881
August 7, 2007, 09:35 PM
Can someone make a diagram or something showing a line representing poi from the sights and then a line from the barrel representing poi? I'm such a visual learner that I can't read it and get a picture of it in my head.

Kilgor
August 8, 2007, 04:04 AM
http://www.remington.com/products/ammunition/ballistics/

chris in va
August 8, 2007, 04:40 AM
I'm sure there's a better representation out there, but just doing a basic google search this is the best I could come up with.

http://demigodllc.com/~zak/firearms/dial.png

Note how the bullet, being zero'ed at say 300 yards will rise above the sight plane, then drop down to 'zero' at 300, hitting the target.

Similar to throwing a rock. You can't just throw right at the target, it has to arch due to gravity.

JohnBT
August 8, 2007, 10:20 AM
http://www.madsci.org/posts/archives/feb2000/951834137.Ph.1.gif

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