What's going on with our ACOGs and the BDC?

[Wake27]

Been shooting the ACOG at work a lot lately and several of us are having a weird issue that I can't understand. We zeroed them on a KD range at 100m for POI to be at the tip of the chevron, as it was designed. However, the past few days when working the normal Army pop up ranges that go from 50-300m, there are a solid number of us (at least 5) that have to hold significantly low for almost every target. My understanding of the 100m zero is that essentially any hold should be high. We changed elevation by about 1000 feet from our KD zero to the pop up ranges, would the atmospheric differences there be enough to cause this? I didn't think so but I can't figure out what else may be the reason besides some severely underpowered M855.

[tigershilone]

So your are aiming under target to get hits? That would imply that bullet speed is higher than normal, right?

You need to chronograph to find actual fps. Without that info and the bullet weight you don't know what is going on.

This is THE reason I don't like BDC reticles, if speed or weight is off by a minor amount then the BDC reticle is not accurate. You are stuck with one bullet weight at one speed to match the BDC.

Hopefully it is just a hot batch of M855

[HCM]

Are you shooting M855 or M855A1?

[Wake27]

So your are aiming under target to get hits? That would imply that bullet speed is higher than normal, right?

Yeah that would actually make sense, not sure why I was thinking it was underpowered. I wonder how hot a lot can be and still be considered in spec.

Are you shooting M855 or M855A1?

Regular M855.

[ExplorinInTheWoods]

When you guys go to M855A1 you'll be off. We used that for our division marksmanship team and using the normal sight in method with an acog it was nearly 12 inches high from point of aim at 300m.

[17K]

Zero it at 300M and see where it hits at all other holds and how it aligns with the actual range.

Zeroing the middle of the BDC spreads the error and it's not off as far.

[Pappabear]

In my experience with ACOGS, you have to fit every lot of AMMO with the BDC, however, I am a fan of ACOGS big time. If you know your ammo and your ACOG, it solid optics. I like to zero at 400 or 500 and see where I am ? I prefer to be on point at 500 than 100, because I will hit at 100 wish in reason and know that hold off.

Best of luck.

PB

[Failure2Stop]

My immediate checks would be the mount and zero confirmation.
You'd have to break the laws of physics to get the projectile to deviate from the ballistic path enough to overfly a giant pop-up.

Agreed with Pappabear on zeroing at a farther distance: better to zero at 300 with the tip of the vertical post than 100 with the chevron, as long as you can accurately compensate for wind effect. The longer zero will balance out the BDC, but that's not the issue you are having.

[Wake27]

Mine was definitely snug, that was the first thing I checked. It'd be extremely difficult for us to zero at 300m here, but I do wish we had been able to investigate the issue more. It was kind of decided to be a little bit of several different things, almost like tolerance stacking, but I'm not entirely convinced.