I used a manual hacksaw to cut 5mm off the length. I went with this so I could go nice and slow and get the end good and straight. Then I smoothed the end, chamfered the circumfwre ce, and used a Dremel tool to cut the slot in the middle of the end. After unwrapping the tape, I sprayed all the parts down with alcohol and wiped it with clean paper towels. After the machine work, the piston ended up being almost exactly 6 mm shorter, which was my arbitrary target.