I have a factory bcm upper and the aimpoint I have on it is zeroed I also have a pair of Daniel Defense back up irons and noticed to co witness them with the aimpoint the rear sight must be drifted almost all the way to the left. Is this normal or what could be wrong?

most likely one or both of the back up sights are not centered on the rail.... is the top rail of the fore end aligned with the top of the receiver?... if so, and you can find access to a milling machine, have the milling machine operator center the upper, put an indexing point in the mill, center the point off the top of the receiver and run the table the complete length of the rifle to see how much difference there may be to the location of the center of the rear sight, and the front sight

Its the buis on the rail and the rail is centered the sights on another rail is centered fine. So I guess its just the rail?

Which Aimpoint is it?

Zero the sights by shooting the gun.   Preferably with the Aimpoint removed, but if you think you're going to aim through the optic, then zero with the optic mounted.

Don't even try to cowitness the irons and the dot.  They don't work that way.

Your handguard might not be pefectly lined up with your upper. Your barrel might not be centered. If it's a really big deal, I would try maybe lapping the front of the upper flat.

Let me guess.

It’s a T-2.

If so, normal.

I have one that actually aligned perfectly with a dot and one about 1.5mm "off". Just plain luck as I see it. Too many factors with OTS stuff to have any concern. I zero the hard sights and I zero the optics. If they fall in love, get married and become one, that's dandy. Otherwise, no biggie.

Everyone is different and requires it's adjustments

Topic Moved

Even when zeroed at the same distance, which usually results in 'co-witness', the fact of 'co-witness' is of no consideration. This is because only an idiot would use the irons and the dot sight at the same time. Once the dot is sighted in, it doesn't matter where it appears in the field of view - its still sighted in.

Let's see,...

Turn off the red dot.

Zero your iron sights.  Be sure this is done with a properly - head position on the stock, front sight centered in the rear aperture.

Zero your red dot.  Be sure this is done with a properly the key being your head's position on the stock.  Use the rear aperture as a guide to keep your head properly positioned.

Record the sight settings (write them down)!

Do not make any further adjustments, as your two sights are now zeroed.

Turn off the red dot.

Aim using the iron sights.

Turn on the red dot.  Keeping your head properly aligned using the rear aperture as a guide.

Note the position of the dot relative to the front sight.

Wiggle your head (cheek weld) a tiny bit and note how the red dot floats over the front sight and the target.  That is how sensitive your aim is to head position.  If you want to shoot well you MUST have a repeatable head position on the stock.

Both sights are properly zeroed