Quote History Quoted:
Ditto.
Make sure hammer is cocked or safety will not rotate.
Make sure lock washer is on the grip screw.
Make sure grip screw is not longer than one-inch. I like 3/4 inch better myself.
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Ditto as well, but to add,
Stock pistol grip, or after market???
If after market, then does sound like the channel for the spring in the grip is too shallow, and it is causing the selector detent spring to coil bind out.
Loosen the grip screw a few winds to drop the grip down just a hair, and see if the selector is still having problems.
If it does, then may have to deepen the spring channel a touch with a drill bit.
Also, with the trigger out, install the grip with just the grip bolt and washer, then take a look at the bottom of the lower receiver void. The tip of the screw should not be protruding up in the the lower receiver cavity. If it is, and you are sure that you installed the grip bolt washer, then you may need to shorten the grip bolt too.
So you can check to make sure that you have the correct spring installed in the correct areas.