Put the new GT bottoms in and drill each hole half way thru, then turn over and drill thru from other side to meet the first ones. If you try to drill all the way through from one side I GUARANTEE that you will miss the hole in the mag on the opposite side!
This is done best with a drill press running on the slowest speed. I use an electric screwdriver fitted with a drill bit myself. for a good tight fit to keep the pin from falling out, it helps to drill the hole in the wood slightly smaller than the pin; but not too small or you will split the wood.
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In swapping original bottoms, find one that comes closest to lining up, you may have to shave off a little wood here and there. Then take a pin punch of the correct size and line the holes up. Insert the mag pin as you remove the pin punch to keep the pin lined up with the holes.