Thanks! It's a one of a kind so far as I know... don't know anybody else with exactly that set up.
I thought about that when I couldn't get it to group... receiver flex, the rail, the mount. I don't think the difference in groupings would be that dramatic though from one ammo to the next if it were just the flex vs bullet weight. Especially at 100yd like you say. Besides that, it's actually a pretty heavy, well built little beast with all the wood (like 11lbs w/empty mag). Reinforced trunnion, thicker gauge metal receiver and I think the rail actually helps to stiffen it. Low recoil, adjustable gas system... I don't think there would be enough flex to move my poi 5" back and forth across the target.
Like I said, I won't know for sure until I experiment with some heavier ammo of better quality. Next time I sit down to see how tight I can get the groups... I'll play around with removing the suppressor. There's a flat spring on there that keeps it from being 'wobbly' like your typical AK attachment... so it should remain accurate I should think, just like the AR's with the birdcage's are.