On my R9, I can apply a slight lateral pressure on the barrel while cocking, and the horizontal lock up, measured in front of the breech seal with a feeler gauge, will shift by .001”. This translates to a 1” shift in the POI at 25 yards. So a laterally neutral hold on the barrel while cocking is essential to my accuracy. But even so, the horizontal lock up will shift slightly over the course of a shooting session, often moving the 25 yard POI 1/2” to the left or right. This shift can happen slowly, so the groups are okay, but they "slide away" from where I want them. Unlike the vertical lock up, the horizontal lock does not rely on a solid steel to steel contact. Instead, it relies on the pivot shims to provide a stabilizing force. In my case, the breech block forks are not parallel, and the shims are pinched together at the forward edge, with little contact at the back edge of the shim. This “point of compression” is less secure than if the shims were in full contact with the breech block. I think this is why I’m seeing horizontal shifting in POI, while other shooters not so much. If this analysis is correct, then a possible solution would be to fix a shim to the inside of one fork, just in front of the breech seal. Then intentionally apply a lateral force while cocking the rifle, and “forcing” to breech block against the new shim. This is a pretty radical modification to an expensive gun. I’m looking for a better idea. What do you think?
here are attachments, i think.