This is the last flip cap you will ever need! Virtually indestructible, this cap will fit snugly on nearly all Vortex Optics and features a snap flat spring that will keep your line of sight unobstructed. The E-10 fits all Vortex Riflescopes except the 1-inch tube Viper model VPR-M-01BDC, VPR-M-04BDC, VPR-M-03BDC and Viper HS 4-16x44 model VHS-4305 and VHS-4304. Outside diameter: 41.5-46 mm | 1.6-1.8 inches Vortex Defender Flip Cap Eyepiece (40-46mm) E-10