I believe the reticle challenges are not dependent on the weapon, just the specific scope. For instance when I put a VLK on my AMR and go to the reticles, it says I have "129/200 kills with the Scout Combat Optic". When I do the same for the AX-50, I get the same 129/200 kills message about the Scout Combat Optic. It also says that when I complete the 200 kills, it "Unlocks this reticle for all weapons using the ACOG scope".
So these aren't weapon specific. If there's a reticle challenge requiring you to use the Solozero NVG, that scope is only available for AR/SMG/LMG/DMRs.
I could be off-base here since I haven't given any effort to reticle challenges yet, since the only optic I use is the VLK and the base reticle is the best one anyway.