Ballistics Question - Weird observation

[Cowboybebop]

So today I'm sighting in a 20" AR in 5.56 with an ACOG and want to be zeroed at 50 yards.

According to my ballistics chart at this zero a 62 grain M855 will be 1" low at 15 yards. So I decide to sight in at 15 yards and get my hits 1" low from bullseye. I'm hitting perfectly 1" low at 15 yards, so I move the target out to 50 yards and fire a group - this group should be dead nuts at this range according to the chart. Instead it's about 4" high.

What gives?

 

[pblanc]

I think you entered the wrong height above bore for your optic. I don't own an ACOG but I believe that the height of the optical center above the top of the rail (not bore) is about 1.45". But the height of the top of the rail above the bore on an AR is typically around 1.2" giving a total mounting height of 2.65" or so. Is that what you imputed?

When I zero a my ARs at 50 yards using iron sights with a height above bore of around 2.6" or a red dot sight with exact co-witness, using M855 62 grain I expect to hit a little over 1.2" low at 25 yards. At 15 yards it would be more like 1.75" low.

 

[Robert]

My rifle hates, HATES, that crap. Best I have ever got is 4-6" at 100y. 55gr, 69gr and 77gr all shoot fine. But that 62gr stuff is just junk in my rifle. My 20" has expensive taste it loves 77gr...

 

[Cowboybebop]

I think you entered the wrong height above bore for your optic. I don't own an ACOG but I believe that the height of the optical center above the top of the rail (not bore) is about 1.45". But the height of the top of the rail above the bore on an AR is typically around 1.2" giving a total mounting height of 2.65" or so. Is that what you imputed?

When I zero a my ARs at 50 yards using iron sights with a height above bore of around 2.6" or a red dot sight with exact co-witness, using M855 62 grain I expect to hit a little over 1.2" low at 25 yards. At 15 yards it would be more like 1.75" low.

Interesting. I'll double check that! Thanks.

 

[Varminterror]

Optic height above bore. That’s the culprit.

 

[jmr40]

If you want it zeroed at 50 yards, zero it at 50 yards. Figure out where it hits at 15 yards - infinity by actually firing and see where it hits.

 

[MEHavey]

ACOG height above bore is 2.7"

If you want its first zero at 50, set 15 yards to impact at ~2.7" (low)
Second zero will be ~190yards
Point blank +/- 2" from 30 yards to 225.




* (assumes 63gr Sierra SP Spitzer at 2,800fps)



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[Steve S.]

As stated, shoot at your desired zero range; the math is good for close, the holes in the paper are absolute.