Some of the scopes are clipping and others are floating above the rails.
Examples of cliping and floating scopes shown on the MK18 Rifle.
https://steamcommunity.com/sharedfiles/filedetails/?id=1793475303
MK18 Elcan sight - Clipping
https://steamcommunity.com/sharedfiles/filedetails/?id=1800714408
M16A4 Elcan sight - Correct
https://steamcommunity.com/sharedfiles/filedetails/?id=1800719693MK18
MK18 Kobra Sight - Floating
https://steamcommunity.com/sharedfiles/filedetails/?id=1800710135
This should be a relatively simple fix, by adjusting the y-axis of the scopes individually for each weapon.