The 21 bolthead will NOT work in a G3/91 in a semi gun.
The back half of the 21 BH is flat. In a semi only gun the hammer postion is farther up than FA and the hammer hits and locks the carrier back. Can only be fixed by disassembly. In FA(exception) the hammer locks on the sear and is only released to fire either way when the carrier is past the hammer.
A G3/91 bolthead will not work in a 21 period because of the wider teeth
to pick a round off the mag. These wider teeth will not pass through the 21's upper shell plate. Also the back half is not flat for the 21 ejector position.
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