There's a reason you test for gas key alignment with your receiver upside down. When you are actually
shooting your rifle, you'll have a magazine inserted, which puts upward pressure on the bolt carrier.
You shouldn't be surprised that the bolt carrier moves a little when the hammer slams into the firing pin, which in turn transfers force into the bolt carrier, especially when there's no magazine inserted.
Also, keep in mind that when you're actually
firing the rifle, the chamber pressure is high enough that the bolt is friction locked in the barrel extension. The bolt is not going to rotate and unlock until the bullet has left the barrel, assuming everything is operating correctly.
Your worry about slight carrier movement while dry firing hurting accuracy is overblown when you consider that the entire bolt carrier group and buffer are slamming back and forth every shot.
And if it still really bugs you that the carrier moves a teeny bit, try it with an empty magazine inserted.
Edit:
I also couldn't help but notice that you're using button head philips screws that are too long in your CMR rail in the video. The correct replacement screws can be found at McMaster, 7/8" black oxide alloy steel flat head socket cap screw, part number 91253A198. Try this direct link, or just search for the part number on their site:
http://www.mcmaster.com/#91253a198/=oj94yw.
They also sell special high-torque allen keys that won't strip out the socket cap as easily, for only 26 cents apiece. Part # 71285A152 or link:
http://www.mcmaster.com/#71285a152/=oj960c
You definitely should not be using a button cap screw on the CMR, because you're only contacting the outer rim, which is putting undue stress on the fastener and also can damage the rail. You also want to use alloy steel for strength over stainless, and the black oxide coating reduces the chance of rust.