Change of zero with shift in body position

[Old Glock guy 1,127]

I have zeroed the Primary Arms RDS on my Century Arms AK at 50 yards from a seated position on a sandbag rest, to the best of the ability of my old eyes.  

I have noticed that when I shoot from a prone position, my group remains reasonably tight, but it shifts down and to the left.  This happens consistently.  At a recent AK class, we started from a prone zero.  I foolishly adjusted my sight to move it up and right.  For the rest of the class, from mostly standing positions, I was shooting high and right.  

I'm guessing that somehow my head position in prone moves relative to the sight (I struggle to get it lined up from prone), and exceeds the adjustment for parallax.  (Does that even make sense?)

Wondering if anyone can shed some light on this.  

[Shane45 807]

http://www.breachbangclear.com/parallax-free-isnt/

 

[High Exposure 5,664]

Even with "parallax free" optics such as red dots (nothing is truly parallax free) you will get a shift of zero with different body positions.

Remember, a zero is a living thing. You can spend all day at the range chasing a zero and never get it.

Consistency and centered are key.

[GRIZ 3,369]

1 hour ago, Old Glock guy said:

I have zeroed the Primary Arms RDS on my Century Arms AK at 50 yards from a seated position on a sandbag rest, to the best of the ability of my old eyes.  

I have noticed that when I shoot from a prone position, my group remains reasonably tight, but it shifts down and to the left.  This happens consistently.  At a recent AK class, we started from a prone zero.  I foolishly adjusted my sight to move it up and right.  For the rest of the class, from mostly standing positions, I was shooting high and right.  

I'm guessing that somehow my head position in prone moves relative to the sight (I struggle to get it lined up from prone), and exceeds the adjustment for parallax.  (Does that even make sense?)

Wondering if anyone can shed some light on this.  

How much is your zero off at what range?

[Nucrunner 75]

When I shoot CMP with an M1 I need to go up 4 clicks when I go from off-hand to prone. Similarly, this past weekend I wore a padded vest for the first time this year, that kicked my groups to the left also....

[Old Glock guy 1,127]

On November 18, 2017 at 3:40 PM, GRIZ said:

How much is your zero off at what range?

OK, good question. Yeah, we're not talking fractions of an inch here.  At 50 yards, I would guess the zero moves about 6" down and 6" left.

Hey Shane45, good article. 

And thanks also to everyone else who chimed in.