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Tell us more about the rifle. Barrel length, length of the gas system, what buttstock (receiver extension), buffer weight, etc.
With short -M4 length - the pressure at the gas port is approximately double what it is on a 20" rifle. Also, since the bullet passes the gas port so much sooner, the cartridge case may still be obturated (still expanded against and gripping the chamber wall) when extraction happens, or maybe tries to happen, and exactly this is the result.
Where did he get the rifle? Who built it?
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