RDS Parallax Effect on POA

[SunnySky]

I mounted a 4-MOA Hiram RDS on my 9mm PCC and brought it to range to sight in. To keep the stock iron sights as backup and more importantly as a reduction of astigmatism effect, the RDS is co-witnessed with them.

During zeroing the RDA at 25 yards, I noticed something interesting. Although trying to keep my cheek weld position stable, any slight head movement will result in RDS moving relative to the front sight post. This RDS movement can easily exceed 1" on the target at 25 yards. During the whole sight-in process, I looked through the rear peep sight.

My 5-shot grouped around 2" at 25 yards. I feel more than half of the spread comes from RDS movement.

All RDS' are marketed as parallax-free. I know no optic sights are completely parallax free, but could it be so bad that causes 1" at 25 yards? Or maybe I missed something more important here, such as my astigmatism?

[Scotty]

What does the gun shoot with iron sights or a scope? "Feel" is meaningless. You should know what the gun can actually do.

No red-dot is completely parallax free. With a collimating lens, the parallax is essentially set to infinity. The tolerance with the assembly will cause it to vary slightly, but it is biased towards further away than 25 yards.

With a 4 moa dot, you also introduce errors in the sight alignment because it is a covering a 1 inch circle at 25 yards. Instead of zeroing to the approximate center of the dot, zero to the 12 o'clock position of the dot, much like zeroing an iron sight front post.

Lastly, the purpose of red dot sights is for target acquisition speed, not precision.

[SunnySky]

With a scope on a rifle bench with a sandbag support, I typically got 1" grouping or slightly larger. Of course, it varies with ammo and from group to group. This time I shot it with a sandbag support on a handgun table, maybe resulting in less stable holding. RDS size doesn't help either, like you pointed out.

After more experiment, I think I have found the reason. Even though looking through the peep sight, a slight head movement (on the order of a millmeter?) will shift my iron sight alignment. The relative movement on the target between the RDS and the front sight post is caused by the error in my iron sight picture. The RDS still holds the same target position.