I think you're trying to accomplish something extremely difficult, bordering on impossible. I shoot a GAP .223 bolt gun from time to time in a local 300 yd "Reduced Palma" match, F-TR division. It's not really a Palma match, but that's what it is officially called. We shoot 3 x 15 shot matches, with the NRA MR-63FC target that has a 10-ring of 0.95 MOA, and an X-ring of 0.47 MOA. Using a Berger 80.5 gr Fullbore load that is sickeningly accurate, my scores over the last couple years average above 99% (at or above 446/450), usually with X-counts in the mid 20's out of 45 shots. Although I can clean a single match fairly often, and on somewhat rarer occasions have cleaned two matches out of three, I have yet to shoot a 450/450 with any of my F-TR rifles. With that setup, I am not shooting sub-MOA for 15 shots at 300 yd, let alone 0.5 MOA. And that is with a bullet having a much higher BC than does the 77 SMK, and is therefore much less affected by wind.
The best shooter (by far) in these matches has shot several 450s with his 6BR F-Open gun, with X-counts typically in the low to mid 30s. This individual has finished well into the top 10 in the US F-Class Nationals two years running. My point is that he is not shooting 0.5 MOA at 300 yd using a world class F-Open bolt gun. Granted, these are 15-shot groups instead of 10, but it's at 300 yd instead of 400, and I think you get the point. Nothing wrong with aiming for the stars, but I think you will be disappointed if you expect to shoot 10 x 10 shot groups at 400 yd at 0.5 MOA with a .223 semi using 77 gr SMKs. Even the slightest breath of wind will move those a lot at that distance. Frankly, I think the shooting you showed in those images is phenomenal as it is.
Certainly some of the reloading tweaks mentioned such as hyper-precise powder measurement, and extremely exacting brass prep and other reloading practices may allow you to improve that ever so slightly. I also weigh powder on a very accurate laboratory analytical balance, typically to less than half a kernel variance. If you do the calculation you will find that half a kennel variance in powder weight shouldn't change velocity by more than about 1 fps. My .223 loads typically show ES values in the 15-18 fps range, so obviously the variance in velocity is coming from something other than the precision with which I weigh it out. My point is that there are other limiting factors inherent in what you're trying to do that will likely make it almost impossible to reproducibly shoot 0.5 MOA for 10 x 10 shot groups at 400 yd. In general, semiautos are inherently more difficult to shoot with the same precision as a high end bolt gun using a well-tuned load. So I'd say you've already accomplished a great deal. Just out of curiosity, have you tried shooting it with the gas system shut off and single feeding? Sometimes that can make a difference. In any event, great shooting. Nothing wrong with trying to tweak things to see if you can improve it even more, but I certainly wouldn't be too disappointed if something around 1 MOA is the best you can get out of it for 10-shot groups.
