Aimpoint T1 zero shift with Iron sights?

[NYH1]

Not sure if this thread should be posted here or in the AR General or Technical sections but here go's.


I put a new upper (BCM receiver and Colt 6720 barrel w/FSP) on my wife's Stag M4. I put both the Aimpoint T1 (4 MOA dot) which is in a American Defense QD mount and Danial Defense fixed rear BUIS we had on the Stag upper on this new setup.


I went to the range yesterday to zero both the AP and iron sights (50/225 yds. zero w/Federal 62gr. Fusion). Zeroed the AP first, then went on to the iron's. I started zeroing the iron's looking through the AP with it turned off. The iron sights out of the box were about 4 inches to the right with the elevation being good. Made the windage move to my zero point and all seemed good, thought I was done.


Then I removed the AP from the rifle to shoot it with just the iron's. I fired a few rounds and they were about 4 inches to the right and 4 inches high. So I put the AP back on with it still turned off and fired another group, it shot back to my zero point. Took the AP off again and the same thing happened, it shot 4 inches to the right and 4 inches high. The AP held it's zero point every time I put it back on.


I have a Aimpoint PRO on my Colt (6920). I shoot it all the time with my AP on using the red dot, with it turned off looking through the AP using just my iron sights (Troy rear flip up and FSP) and with the AP removed from the rifle. It shoots to the same zero point in all three mentioned conditions.


So my question is, is the T1 setup different then the PRO, making it change the zero point when looking through it compared to not looking through it because it's removed from the rifle? It doesn't make sense to me. However, I'm no SME on anything let alone optics.


Thanks, NYH1.

[WS6]

I have had this happen with the T2, but not T1.

[NYH1]

I have had this happen with the T2, but not T1.

I wonder what causes it to do it on some but not others. I never heard about before it happened to ours.

NYH1.

[Deadsquiggles]

I’ve seen it with low end red dots. I read that it was due to cheap film on the lenses and the angle of the lenses or at least something to that effect.

[ggammell]

Anytime you put a lens between iron sights your zero will change. T1 is a smaller lens that the pro with difference curvature affecting the light in a different way than the PRO.

[NYH1]

Yeah, I thought maybe having the optic with multiple lenses in between the front and rear sights might have something to do with it.


Thanks, NYH1.

[calvin118]

I too have experienced this with the T2s but not the T1s...

[scooter22]

There are lenses between the sights...


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