Michael, 2 random thoughts spurred by the obvious vertical dispersion:
Forearm touching the receiver is a no-no
Barrel may have uneven contact against the front face of the receiver
The apparently vertical-only dispersion indicates, to me at least, that the underlying problem may be mechanical and from the outside rather than from anything within the barrel or ammunition itself. That is, uneven vibration of some sort such as an uneven connection between the various parts of the rifle, only manifesting itself under certain conditions.
Like I said, random thoughts.
Regards, Joe
