The range finder is described as:

Increases your accuracy and range

This can be scooped up in south-west Espiritu Santo, in Bahia Floresta Village swarming with cartel. A Rebel Radio side mission is nearby. The accessory case is upstairs in the room above a garage in a house in the middle of the village.

I approached this via the road from the north-west and stopped to surveil the village with my drone. It was the middle of the day and this showed there are about 20 sicarios in and around the buildings in the village. Although the accessory case is in the middle of the village, it’s at the edge of the cartel presence. At night, I expect you could sneak in and pick this up, but when I tried to take out the sniper on the balcony, my cover was blown and I ended up just going in full steam and killing everybody. I recommend a little more caution.

You can equip the range finder to your gun’s rail to improve the range stat of your gun, but bear in mind the additional weight at the front of your gun reduces accuracy by a fair chunk and handling by a small amount.

I’m not certain what the range stat really denotes, but I imagine it relates to your bullet drop, or how far your bullet travels before dropping off. I’ve noticed for instance, that you can forget about shooting distant targets with an SMG unless you lob the bullets, because they don’t have the range of a sniper rifle. So perhaps this will help if that’s what you need, but personally, I’d just swap out for a more appropriate gun.

So I don’t have a real use for the range finder and I never equip it, but perhaps your mileage may vary and you’d like to try it for yourself.