I've received my tax stamp and want to toss on a stock but I can't quite get the end plate off. I'm pressing the pin fully, hammering the bottom of the plate so that it slides off, and it won't even move a little bit. I've thought about penetrating oil and then trying to see if it'll move. I've tried different hammers and I basically want it off since I'll never put it back on. What am I doing wrong?