Peening is a technique used by pistolsmiths in conjunction with slide squeezing to tighten the slide/frame interface.
The frame is supported in a vise and spacers are placed in the rail slots. A ball pein hammer is used to hit the top of the rails multiple times, thus lowering them. When the rails are lowered the desired amount, the swelling caused by the peening is dressed. The peened area of the frame rails is then filed and/or ground to fit the slide rails, followed by lapping and polishing of the bearing surfaces.