Scale the reticles realistically. That means whatever you see when looking through a real ACOG should be similar to what you see in game. ACOGs also have a MASSIVE field of view and crystal clear glass. That means no "toilet paper tube" model where you slap a reticle on a long tube (See COD4s ACOG; that is what NOT to do). Eye relief and eyebox effects would also make the game more immersive. Modern Warfare 2019 implemented those and some optics look almost photorealistic.
Here is what a real ACOG looks like on an M16/M4: (EDIT: Apparently the model of ACOG used in Fallujah did NOT have the long black horizontal lines either side of the Chevron... I can't find any pictures looking through the 2004 ACOG)


ITL MARS optics were also used by some, for those saying it was only ACOGs and irons. (Image below is a Marine in Fallujah using an ITL MARS sight)

Good games to take notes off of are: Ground Branch, Modern Warfare 2019 (it has amazing optical distortion, eye relief, and fisheye effects), Arma 3's RHS ACOG, and Squad.
Also, if a dev reads this, could you tell me if you are using Picture in Picture or normal 3D scopes? PiP may be unattainable for consoles. But you can add a fisheye filter like MW 2019 and achieve a similar effect.