vertical stringing why

[stangguyaz]

Went shooting today to test some extraction issues with my AR10 I shot this group. a lot of my groups look like this vertically stringing. The shooting range right by my house has concrete T shaped benches that make it really hard for me to get directly behind the rifle. They do not allow prone shooting on the main range so I am stuck with seated position. any ideas on what I am doing wrong ,cheek pressure trigger pull. I shoot off of a front sand bag and a rear squeeze bag Thanks

 

[Sterling Shooter]

some of the possible causes of vertical grouping:

1. Poor follow through
2. Changing sight picture
3. Changing sight alignment
4. A very low position
5. Failure to maintain proper eye/sight alignment
6. Varying the butt position in shoulder
7 .Not enough head pressure
8. Position of left elbow wrong
9. Changing distance of eye to sight

 

[stangguyaz]

Probably doing about all those wrong I have a really hard time getting comfortable at the benches