IIRC, your pictures indicate this occurring with the round feeding from the right side of the mag. Is this the same case in all the malfunctions?
Assuming the rifle has been supported with the same firmness throughout your experiences, locking back to the rear seems to indicate you are getting enough gas to ensure the bolt head is picking up the round.
If the bolt head, and not the bolt carrier is picking up the round, the easiest way for you to get the dented brass you describe, is for the round to be kicked upward, into the top of the chamber during feeding - often this is a feed ramp issue.
Since you have M4 feed ramps I'd check to ensure that the M4 cuts in the upper blend nicely with the M4 extension.
I would also go with an H or H2 buffer.
Patriotism means to stand by the country. It does not mean to stand by the President... - Theodore Roosevelt, Lincoln and Free Speech, Metropolitan Magazine, Volume 47, Number 6, May 1918.
Every Communist must grasp the truth. Political power grows out of the barrel of a gun. Our principle is that the Party commands the gun, and the gun must never be allowed to command the Party Mao Zedong, 6 November, 1938 - speech to the Communist Patry of China's sixth Central Committee
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