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Discussion Starter #1 (Edited)
Hi guys,
I have a question on sighting in my Bushmaster XM-15 carbine.
I have a quad rail with green laser mounted on left side and a red-dot sight in place of the original rear iron sight. if I use a red laser boresighter and align my green laser and red-dot/front sight on it at 25 yards they are out of alignment at any other yardage. What's wrong here? How do I maintain alignment at unknown distances. I must be missing something. What?

Thanks,
Bigjohn
 

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Drunk Supernova
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6,002 Posts
You can't really without making elevation adjustments at the varying ranges. That is why you have a zero. You set your sights to a known distance. Then through range estimation and known comeup/down clicks you adjust your shots when firing at different ranges.

But one issue that ya will come across with the laser is that it is not only angled up or down to get your POA/POI but it is also angled toward the barrel. So if you fire at another range, either closer or farther away, your POA will be either further to the left, or right respectively. Unless ya have it set directly parallel with the bore plane and only adjust for elevation. But then the laser will always be off to the left of where the round will impact.

Another thing. Ya really shouldn't use a bore sight to zero your rifle at any other range than that rifle's and just as importantly, the particular bullet weight's point blank range. Point blank range is defined as the two points during that bullet's trajectory where they will naturally fall in line with a laser from the bore plane. As the barrel is angled up, and as the sights on the top are above the barrel plane, the bullet's trajectory will cross the bore plane twice. Say you're firing 62 grain FMJ at 3110 ft/sec from a 20 inch barrel with standard iron sights atop the carrying handle, then the point blank ranges will be 36 meters and 300 meters. Using a bore sight at any other range will give you a different point of aim than your point of impact, as will using those two ranges if you are firing a 72 grain boat tail open tip at xxx ft/sec. Or using a different type of sights with a different sight height.
 
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