I have a new build. SPR setup with 3-10M3 on a Larue tactical SPR\M4 mount....
Shooting 75gr BHs....
I'm averaging about 1\2-3\4MOA at 100yards...But when I get comfortable with a beanbag,,,,i average 1\4"....

Anyway...the conditions are terrible with the cold and wind but I have been getting some verticle stringing....I wont be able to verify it until i got out a few more times so the rifle and work things in...

The only thing is that the issue of verticle stringing seemed to appear when I swithed from an A2 flash hider to a MSTN\PRI brake....

What are some possible causes of verticle stringing in an AR?

inconsistent powder charges.

Did you over-tighten the muzzle brake? I think twisting the sh!t out of any flash hider / muzzle break can affect accuracy.

just hand tightened and then another 1\4 turn to index...so wasnt very tight.

If anything when I switched from A2flashhider to MSTN brake..the group went up 1MOA.

Good, I'm just a little unsure what you are trying to say by "the group went up" did they (the hits)get closer together on the target or farther apart? do you see any evidence of gas leaking around the FSB? (or anywhere else on gun.) You also might want to try and lock your rifle into some type of bench rest vice or use something other than sandbags.

no gas tube is ever air tight at the gas block and tube junction....so i cant see where you would get a consistent, repeatble problem there.....
Obviously I need to go reverify it but is there any other possible cause for verticle stringing?

Quoted: inconsistent powder charges.

+1

Let's see, accurate rifle and cold wx.

More clothes, different cheek weld makes parallax error more apparent.

Shooting from different surface, bi-pod bounce. (Assumption)

Lower ammo pressure, different harmonic relationships.

Or, something else.

Rifle accuracy sometimes mysterious.

Regards,
Mild Bill

Eye of newt
Wing of Bat
I hope I hit
where the target's at.

Quoted: no gas tube is ever air tight at the gas block and tube junction....so i cant see where you would get a consistent, repeatble problem there..... Obviously I need to go reverify it but is there any other possible cause for verticle stringing?

Often, vertical stringing is associated with breath control. You mentioned the conditions were terrible. Is it possible the conditions affected your posture, breathing, etc.?
FWIW
YMMV

Quoted: Quoted: no gas tube is ever air tight at the gas block and tube junction....so i cant see where you would get a consistent, repeatble problem there..... Obviously I need to go reverify it but is there any other possible cause for verticle stringing? Often, vertical stringing is associated with breath control. You mentioned the conditions were terrible. Is it possible the conditions affected your posture, breathing, etc.? FWIW YMMV

+1

vertical = breathing
horizontal = trigger control (among others)