[wyvernfist]

http://www.butlercreek.com/productsImages/charts/flip_scope_chart.html

 

[wyvernfist]

AirsoftGI:

"Objective Lens Diameter: 40mm"


Butler Creek:

"Size 17 - Objective (MM) 40.9 - Part# 30170"


However, I can't get confirmation on whether these run tighter or looser, therefore whether to get size 15 or 17 for a 40mm objective..

Edit: size 15 (30150) is 39.6mm so I'm gonna recommend that for now..