What am I doing wrong- alternating group sizes

[Vodak]

I noticed something unusual that I think points to a failure in my mechanics. I took my Vudoo out this weekend to do a bunch of lot testing. When I went back to look at the targets I noticed that I was frequently shooting larger groups with the first five shots, followed by a tighter group with the second.
Shooting at 50 yards off a bipod and rectangular rear squeeze at bag (no ears) from sturdy but not rigid bench. Loading ten rounds, five to the first target, pivot right, and the second five to the second target.
I did have another shooter offer to watch me shooting, and the only thing he noticed was that I was tensing my shoulder in preparation for breaking the shot. I tried to consciously keep my shoulder down, but it didn’t seem to change the pattern in how I was grouping.
Any thoughts on what I might be doing to cause this?
Groups are all five rounds each, left target first, then shift right, usually without breaking position except for the slight pivot.

 

[usafa77]

To me a bench is not a good spot to evaluate fundamentals.

 

[HMRamateur]

Hard to say, but from my experience, my horizontal stringing usually came from too much cheek pressure on my stock, and vertical stringing was either rear bag or bipod related. Could be a combination of both. Also, a wobbly bench would have an impact.

 

[Defender32]

What I have learned in all my 50y dot drills is this.

Stringing L-R can be the way you have the rifle slightly canted.
Vertical stringing can be your breathing, rear bag squeeze and trigger press.
*** You can also see slight variations in these with velocity differences in the ammo. ***

One other thing it can be is your positional zero from a modified prone position is off. I'll verify my zero from a Solid prone or Solid Bench position. Then I want to see what it is from a prop. 95% of my shooting is from a prop so that needs to be a LARGE factor for me.


Look at your groups over 10 rounds to 1 target. Looks like you have .2 dialed in so you're not taking out your aiming point. This is also good once zero is verified. Try to be SLOW and focus on a solid position with focus on breathing and breaking a clean shot. Do this with an empty chamber for 10 -20 rounds if needed.

The vast majority of your groups look great. Most are all sub .5 groups and we all strive to have them better than that. Not ideal shooting conditions can induce larger groups. Just continue to shoot and focus on breaking great shots.

 

[Jack_L]

I think you're mostly seeing the average of the rifle/ammo combination.
I do everything with 10-round groups to identify the true dispersion of the system through a stage. Even then I will get variations in group size, from about 0.3"-0.6" (I include "fliers"), with an average of about 0.4" in my current primary rifle.
I try to let the barrel cool after I do the initial fouling of 20 rounds or so, and try to not let the barrel get more than 20 degrees or so above ambient. I will let the barrel come back to ambient and once I have a few groups downrange to check for "cold bore" fliers. I have had some barrel/ammo/cleaning process combinations that would show it when the barrel cooled. I also track group shift over 10 groups of 10 rounds, as it is common to see about a 0.1-0.2 MRAD shift between the first groups after fouling to the 10th group. Knowing where this happens, and accounting for it during a match can avoid a lot of frustration.
Wind does have an effect at 50 yards, resulting in not only left to right variation but also in elevation. I will shoot some groups intentionally during wind peaks to track that difference. A 1 MPH full value wind will push POI almost 0.1 MRAD at 50 yards (it is very easy to not notice a 1-2 MPH wind) and 7 MPH will change elevation by almost 0.1 MRAD. Not trying to lecture anyone on wind, I'm just pointing out that it takes very little environmental input to affect your group sizes at 50.