Glock Rear Sights - Markings
Left to Right: 6.1mm 6.5mm 6.9mm 7.3mm
6.5mm is standard on all 9mm, .40S&W, and model 33
6.9mm is standard on all .45acp, 10mm, and models 31 & 32
Changing the rear sight to the next smaller or larger in
the series will move the bullet impact about 3 inches at 25 yards