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Old 01-25-2021, 06:24 AM
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Nomercy448
Nontypical Buck
 
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Join Date: Oct 2009
Location: Kansas
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If you are putting a laser boresighter dot just a little low and left of the crosshair when viewed at a distance of 20ft, then your bullets are hitting high at 100yrds. Way high.

This is a really, really common mistake folks make when using laser bore sighters, especially in AR-15’s.

An AR-15 will typically have a 2.4-2.6” scope height over bore. When you bring the crosshairs to a laser boresighter dot at 20ft, you’re drawing a TIGHT X between the line of sight of the optic and the bore axis - you’re dropping that 2.6” within 20ft. Extending that X to 100yrds (280ft past the intersection), your bore axis is now pointing 36” above target. A 150grn SST at 1950fps will fight about 4” of absolute gravitational drop by 100yrds, so I’d bet you’re striking somewhere around 32” high at 100yrds, quite a bit over the top of your 18-24” paper.

When I laser boresight AR’s at short distance (typically 30ft across my shop), I use a printed page with two target centers 2.6” apart, hanging vertically. I place the laser dot on the bottom target, then zero the crosshair to the top target. I’ll dial up the number of clicks necessary to accommodate a 100yrd drop - typically ~1.5-2MOA for 5.56/223 (6-8 clicks on a 1/4MOA or 1/4IPHY scope), then shoot at a 25yrd target, which should print about 1.5-2” high, or shoot at 50yrds, which should print about 1/2” high, then move to 100yrds.

But trying to center the crosshair on the laser dot at short distance just doesn’t work.
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