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Posted: 2/29/2016 3:48:08 AM EDT
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Sorry not quite sure which forum this goes in....
I want to do a 50 m zero on my new AR. Iron sights (LMT 6/3 drum). I am reading a lot but still fairly confused, please let me know if I am getting it right: 1) modify the rear to crank down 6/3 minus 4 2) get on paper at 50 yards (yards meters whatever close enough), with 6/3 minus 4 setting 3) make elevation changes with front sight only 4) once zeroed, in theory I can leave it 6/3 minus 4 and shoot at any distance until the unlikely event I want to shoot 300 yards, then crank it up to 6/3. I think I got all that right... If so, then let me ask the following question: with a 50 m zero, where the trajectory of the round intersects point of aim at 50 & 200, and if I shoot at 100 yards (meters, whatever), there is NO rear sight drum setting I can make that will allow me to shoot point of aim = point of impact, correct? This is because to do so I'd have to put in some down on the drum and I can't as I am already bottomed out at 6/3 minus 4. If I DID want to allow for dope changes on a 50 m zero when shooting 100 yards I'd have to further modify the drum to allow for, say 6/3 minus 6, then zero at 6/3 minus 4, which would allow me a few clicks down if I didn't want to Kentucky windage it when shooting at a distance between 50 & 200 yards... Right? |
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Quoted:
Sorry not quite sure which forum this goes in.... I want to do a 50 m zero on my new AR. Iron sights (LMT 6/3 drum). I am reading a lot but still fairly confused, please let me know if I am getting it 200, and if I shoot at 100 yards (meters, whatever), there is NO rear sight drum setting I can make that will allow me to shoot point of aim = point of impact, correct? This is because to do so I'd have to put in some down on the drum and I can't as I am already bottomed out at 6/3 minus 4. If I DID want to allow for dope changes on a 50 m zero when shooting 100 yards I'd have to further modify the drum to allow for, say 6/3 minus 6, then zero at 6/3 minus 4, which would allow me a few clicks down if I didn't want to Kentucky windage it when shooting at a distance between 50 & 200 yards... Right? What distance range you are zeroed at is dependent on the ammunition you use and barrel length. For example: M193 out of a 16" barrel zeroed at 50 yards will be: 0" at 50 yards +1.6" at 100 yds +2.0" at 150 yds +1.1" at 200 yds 0" at 225 yds -1.3" at 250 yds -5.5" at 300 yds M855 out of a 16" barrel, 50 yard zero: 0" at 50 yards +1.5" at 100 yds +1.8" at 150 yds +0.8" at 200 yds 0" at 220 yds -1.8" at 250 yds -6.0" at 300 yds a 77 gr OTM at 2730 fps, 50 yard zero: 0" at 50 yards +1.3" at 100 yds +1.2" at 150 yds 0" at 188 yds -0.5" at 200 yds -3.9" at 250 yds -9.2" at 300 yds If you want to go out to 300 yards, you should zero at 300, or adjust 1" high at 50 yards. Just remember at 300 yard zero, at 100 yards you are +3.5" high and the maximum ordinate is around +5.0 inches at 175 yards. If you absolutely have to have sight movement to adjust to 100 yards, you can reposition the elevation index so you have enough travel on the elevation knob to get there. You can zero at 100 yards, then by just moving the elevation drum on the rear sight (not the front sight) adjust to hit a 300 yard target. Now just use the little set screw in the elevation drum to move the drum so the "3" lines up with the index line. |
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Oh, and if you don't know how to reset the position of the elevation index on the elevation drum:
The set screw (A) fits into the indicated threaded hole on the elevation index, the post goes all the way through and engages one of the many holes elevation knob (B). The screw can be loosened with an allen key through the hole marked (C). When you back the screw out it will lock the elevation index at 3/6 (or 3/8 depending on the type of index), the elevation knob can still be moved by the turning the bottom knurled flange. The complete step-by-step procedure is: 1) set the rear sight at 3/6 or 3/8 whichever you have. 2) loosen the screw (A). 3) using the lower knurled flange only, bottom out the rear sight 4) zero the rifle at 100 yards with the small aperture and the front sight only. 5) using just the bottom knurled flange to adjust the rear sight (don't touch the front sight), to shoot and hit at 300 yards. 6) tighten the set screw to lock the index to the knob. Now, the small aperture is zeroed at both 100 (completely bottomed out) and 300 yards (drum at 3/6 or 3/8), and with the elevation drum set at 3/6 (or 3/8), the big aperture is zeroed close to 200 yards.* 
___________________________ * not exactly because the sight is actually set up for a 25 meter zero. |
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In addition to the above required data, one would also need to know the height of the optic or sight being used. Quoted:
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What would the differential be at 25 yards for a 50 yard zero? Thanks. In addition to the above required data, one would also need to know the height of the optic or sight being used. For the above data I assumed standard height AR15 iron sights, since he started by discussing the standard rear sight. (That .040" difference between standard and the "F" FSB isn't going to make any difference.) Red dots and other scopes might be different. 25 yard zero raises the trajectory, the long range zero is around 350 to 425 yards depending on ammunition, etc... |
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Quoted:
For the above data I assumed standard height AR15 iron sights, since he started by discussing the standard rear sight. (That .040" difference between standard and the "F" FSB isn't going to make any difference.) Red dots and other scopes might be different. 25 yard zero raises the trajectory, the long range zero is around 350 to 425 yards depending on ammunition, etc... Quoted:
Quoted:
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What would the differential be at 25 yards for a 50 yard zero? Thanks. In addition to the above required data, one would also need to know the height of the optic or sight being used. For the above data I assumed standard height AR15 iron sights, since he started by discussing the standard rear sight. (That .040" difference between standard and the "F" FSB isn't going to make any difference.) Red dots and other scopes might be different. 25 yard zero raises the trajectory, the long range zero is around 350 to 425 yards depending on ammunition, etc... I try not to assume. Written communication is difficult sometimes, especially with technical information. It's kinda like trying to describe trigonometry over the phone. Just trying to clarify thoughts in communication. |
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What would the differential be at 25 yards for a 50 yard zero? Thanks. It seems like everyone is overthinking your question, perhaps I am underthinking it, .lol. If I understand your questiion, you are asking where you would hit at 25 yards, if you have it zeroed for 50 yards. Is that correct? Depending on your ammo and sight height you will be hitting probably 1 1/2 to not quite 2 inches low at 25, if you zeroed at 50. Here is a link to a post on this board showing different trajectories of different zeros: http://www.ar15.com/archive/topic.html?b=3&f=118&t=495607. |
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