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The Vortex Defender caps have a rubber sleeve that will stretch to fit scopes in the ballpark of its objective number. I own several Defender caps in various sizes for scopes of various sizes, and their website spec's are good in my experience.
e.g. on Vortex's website the Defender Cap "Objective 50" is listed to fit "Outside diameter: 55-59 mm | 2.2 - 2.3 inches".