This Butler Creek flip-up objective lens cover opens at the touch of your thumb and features a watertight friction mount and semi O-ring that securely anchors the cover, sealing out dust and moisture. The objective lens covers feature two finger tabs extending from both sides of the cap, perfect for right or left handed shooters to quickly open the caps for that shot of a lifetime. To select the correct cover, refer to the scope's owners manual or measure the outside diameter with calipers and select the closest measurement from the table below that is .01" under to .025" ( .25mm- .64mm) over the measurement you get. Diameter mm and Inches and Butler Creek #25.4mm or 1" #1 27.8mm or 1.09" #4 30.0mm or 1.18" #2A33.0mm or 1.3" #3A34.0mm or 1.34" #335.2mm or 1.39" #536.3mm or 1.43" ...