This is described in the game as:
A holographic sight. Swap reticles while aiming.
You can find this in central Itacua province at Atolla Farm for the first mission “Amaru’s Rescue”. You’ll kick yourself if you don’t already have this! The accessory case is sitting on a bench outside the farm house, on the eastern wall.
If you have to return to collect this, the eight enemies will respawn and be patrolling the farm as usual, but being Itacua, I don’t need to tell you how to take them out.
The Panoramic Scope offers 1.0x zoom like other red dot and holographic sights, and is very clear. It’s my favourite red dot for SMGs (and for an AR, if you don’t want to use it at range). It has two reticles which can be switched while aiming, but I particularly like its lightweight design which doesn’t obscure your view.