Vortex Optics Diamondback HP 4-16x42 Riflescope with Dead-Hold BDC Reticle - DBK-10019 Reviewhttp://tinyurl.com/lscbfe9 Vortex Optics Diamondback HP 4-16x42 Riflescope with Dead-Hold BDC Reticle - DBK-10019 Tube Size - 1-inch diameter.Waterproof - O-ring seals prevent moisture, dust and debris from penetrating the riflescope for reliable performance in all environments.Fogproof - Argon gas purging prevents internal fogging over a wide range of temperatures.Capped Reset Turrets - Allow re-indexing of the turret to zero after sighting in the riflescope. Caps provide external protection for turret.Side Focus - Adjustment located on riflescope's left side and is easily accessible from shooting position. Provides optimal image focus and parallax removal.