FIlling voids.
I have a Recce gun that I built long ago. It has been updated with a better barrel in a 16″ V7 match barrel (OG was a Larue Stealth Barrel). One Of my buddies at the time was a Master Gunsmith and he milled out a billet upper reciver for the rear anti rotation tabs on one of the old Geissele MK8 rails. The lockup of that upper to rail is about as close as your going to get to monolithic with having a match upper / rail.
Long story short the gun is a 1/2 moa 556 gun. I love it and probably dont shoot it enough. Clearly the rifle could run a 3-15 or 3-18 and be more of a precision rifle. But when it was built it ran NightForce 2-10 that I later replaced with one of the Gen 1 Vortex PST 2-10. I have tried a ton of scopes on this gun with no luck. Something about the 2-10, a crazy big eyebox, and the fact that with the mount, the optic only weigh about 22oz. 3-18 is heavier and and I dont really have an issue running the 2x in a CQB role but the 3x or 3.8x really forces me to have an offset RMR = more weight.
I have a Leupold MK5 3.8-18 on my SR25 and absolutely love that optic. With a T3 reticle it is fast, generally lightweight for that category, and easy to run (great eyebox, dials, illumination, precise clicks, capped windage, and a great magnification dial). Leupold recently came out with a MK5 2-10. But in true Leupold fashion the reticles suck as with 90% of their scopes. Either a simple duplex or some weird BDC and most things are in MOA not mils.
I recently got some time behind the 2-10 and it is a pleasure. The eyebox is massive which is great for a GP or recce gun as you can find yourself in some weird positions. Its a hard scope to understand its use case. Below is a video done by Mark Smith over at JBS Training where he walks you through why you may want the MK5 2-10 over say an LPVO 1-10. I will be all over this optic once Leupold get some decent reticles in it. But the video is worth your time: