^^^what he said. I would also add that you are operating a semi-auto beyond its design tolerances. Excess heat, hammer, trigger reset and bolt cycle time may not synch properly resulting in an improper contact of the firing pin to the primer. Technically similar to a short stroke. A select fire rifle is designed as such, the device you use is forcing the issue by rapidly pulling the trigger not by a bolt driven, properly timed, dropping of the hammer. Just my .02.
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