the only thing that i can think of is either the cut out of the selector plate wasn't done right, or the little selector plate switch is broken.
does it act like it's burst firing on auto or semi? also make absolutely sure that the trigger wont contact the trigger switch body when a full of the trigger is done. i had to glue on a metal shim of aluminum to the back spine of my trigger. it may seem that the switch can take it, but all it takes to snap it off the board is a fast good squeeze of the trigger.
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Weapons Technician / Gunsmith
Don't look at me, I don't know, lol ¯\(°_o)/¯.
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