The screw holes on the bottom are for weights as Buffalobo said. I used them to mount a red dot to mine. You can adjust the trigger pull to less than an ounce pull. The firing pin will contact the barrel if it is dry fired. That can dimple the chamber mouth. Sometimes the mags have to be tuned to the individual gun. There is a very fine wire hairpin spring held in place by the left grip easy to lose. The pin under the barrel on the frame locks the barrel into the frame, push it in to the rear and the barrel will lift up and off. The cutout notches just behind the muzzle are there to mount a muzzle brake.
They can be extremely accurate. Olympic level competition accurate.