Its not simulated in any way that you would consider normal or correct. Like Arma for example. That is pretty good simulation for bullet physics.
However this game is not grounded in realism, so it should be more arcadish/forgiving. It doesnt stop the bullets, but what it does do is 100% change the physics. At that range I dont know what happens. You can have the cross hairs perfectly on the machine and nothing happens.
You can raise the arc of firing and see if bullet drop is the issue and there is no indication that it is. You can shoot 50 meters closer to you than the target and it hits. You can aim directly at it and it will fly over the machines head or to the left or right of it by miles. There is no knowing or predicting from gun to gun. We dont even know if wind speed is a factor because there is no indication that it is or should be.
I highly doubt they included windspeed and multiple zero out ranges per weapon. If they did, we cant predict or know what those speeds or ranges might be per scope/rifle combo. Since I doubt that its that complicated it only leaves one possible option. Machine AI and world effects/feedback in game.
What I think is happening is that we are hitting them at those ranges, but there is no indication that it hit them. They do not or cant respond to damage from a specific distance. This is because the damage a machine has taken is persistent for your entire session. That way when you come across the machine later in the session, its just as damaged as it was when decided to stop shooting it/died last time. I think this is done so that when you die/quick travel or run away, that Machines do not maintain agro to you. Otherwise each time you respawned anywhere in the world, the machines would try to get to your location again. So they set a range at which all machines will ignore the player character. We know friendly fire is on between the machines, so the AI is probably counting the damage source as not coming from the player character.
Mix that with the fact that at range there is no visual or audio indication that you damaged the machine and you can only assume you missed hitting it entirely. Thats probably due draw distance and visual processing savings for the engine and not something done on purpose.
A fix for this would be to increase agro ranges to 2000 meters or more as long as Line of Sight and Sound/Audio indication are feasible for the machines AI to respond to. So if you use a silenced weapon for example and you duck behind cover before the bullet hits, the machine may have a harder time finding you. If you stay out of sight and/or stop making lots of noise, then agro should stop. Since there is no traditional agro indicator/mechanic to drop agro, then I am not sure that can ever happen in the game. They can use the Animal indicator mechanics from Hunter COTW and reverse it. The machines attack instead of run away when agroed. That would require them to add cover values to foliage and would require them to allow players to go prone and hide like in Hunter COTW.
The next part of the fix would involve dropping 100% of agro upon death and then not allowing the player to respawn anywhere within 1000 meters of those specific damaged machines. Even if there is a safehouse within that 1000 meter radius. The only thing that can bypass that mechanic is the radio beacons. To combat people just stock piling them and using them as adrenaline replacements, make the radio beacons take up 3 vertical slots in the inventory.
I dont know how much the engine shares with Hunter COTW, but there is no reason to re invent the wheel when the code already exists. All that it would be would be a behavior change on the part of the machines versus the way Animals react in HCOTW. O and significantly increase draw distance and animations when using the scope so player can get an indication that something happened with the bullets.