Sight a scope for 100 yards using only 25 yards


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Thernlund
November 1, 2007, 06:11 PM
(That should read "yards" in the title. Not "years". Sorry.)

Obviously distance is not relevant when zeroing windage. But I only have 25 yards to work with for elevation. So do I understand this correctly?

If my scope is 3" off the bore and I sight it in at 2.25 inches under the crosshairs at 25 yards, that should be a 100 yard sight.

Ruger M77/MKII in .308 Win.


-T.

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strambo
November 1, 2007, 06:33 PM
Your scope probably isn't 3" above the bore (center of scope to center of bore) unless it is mounted really high. Most 40mm scopes will be about 1.5" and my 50mm objective scope is 1.75" above the bore.

To see different zero distances just plug #s into a ballistics calculator like this one. http://www.eskimo.com/~jbm/calculations/traj_basic/traj_basic.html

Find a bullet from the pull-down menu. Make an educated guess at velocity (or use data on cartridge box or actual chrono data). Set the zero distance to 100yds. Set the interval to 25 yds. Leave the atmospheric data alone for now, it won't effect a 100yd zero anyway, just long range ballistics.

So, I left the sight height at 1.5", selected a Barnes 165 gr spitzer at 2700 fps and got a trajectory of -.6" at 25 yds for a 100 yd zero. So...if you are grouping 1/2" low at 25yds, you should be very close to dead on at 100.

You can also use this to zero so many inches high at 100 to be on at say 300 yds or a maximum point blank range value. For this calculation, it lists the max. PB range as 324 yds (bullet within +/- 5") when zeroed at 276 yds. Zeroing at 276 yds would be a PITA, so enter 276 yds as a zero in a new calculation and look at the trajectory at 100 yds (for this example: 4.2" high).

This won't work at 25 yds though, the distance is too close and the differences in trajectory too small. If you remember 100 yd zero = .6" low at 25yds. Well, with a 276 yd zero entered, the trajectory should be .4" low at 25yds, only 2 tenths of an inch difference, but the difference at 100yds was 4 inches.

Anyway, play with the program, the results are just to get you close, then fine tune at the range.

esq_stu
November 1, 2007, 06:49 PM
Fred's has a 25 meter qualifying target that simulates 100 meters. He throws in a pamphlet that explains how to use the targets plus a lot of other info.

www.fredsm14stocks.com

Thernlund
November 1, 2007, 07:06 PM
Your scope probably isn't 3" above the bore...

Arbitrary number for example purposes.


-T.

KI.W.
November 1, 2007, 07:31 PM
my pistol scope is 3" higher. The Gun is T/C Contender 9mm Luger 10" barrel with "see trough" scope mount. Shootin to 50 yards with iron sights and 100 with 2x scope.

gyp_c2
November 1, 2007, 09:55 PM
That's a great table there Strambo...thx for the link...30-06 Winchester 180gr Accubond Supreme info plugged in looks awesome...;)

rr2241tx
November 2, 2007, 12:11 PM
25 yard range will get you on paper at 100 yards, nothing more. The error in measurement of displacement at 25 yards precludes meaningful extrapolation to 100 yards. And, no, you can't adjust windage at 25 yards either, same problem as for elevation. If you really want to know that you are zeroed at 200 yards, you must shoot at 200 yards, etc., all you get from fractional distance targeting is an estimate of POI and it isn't a very good estimate in most cases. But, if you are satisfied with 1 Minute of Pie Plate, you can set most common 30-06 / .308 based rifle calibers to be 1.5 - 2.5 inches high at 100 yards and hold dead on to 250 yards. If you're shooting beyond 250 yards, you really need to practice because the ballistic trajectory really gets tricky once gravity gets the bullet dropping over that much time.

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