So, first thing, you will need to go to the location in the screenshot below.
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More precisely, this place:
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Even more precisely, and this is optional, I like to do it here (were Rico is aiming).
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Now that you are in position, let's get to how this method works.

So, if you have good sight, you'll notice some military pickup (Chepachet PVD) down the road. If you have eagle sight, you'll see that it has one guy using the mounted gun:
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The trick here is that this guy's view distance ends just a few meters pass the road signs.
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So, what you're gonna do is: get a sniper, get in position, shoot, get away some 6 seconds to cool heat off and for the pickup to despawn, get to the position again, and the pickup would respawn, with the guy in the mounted gun. You repeat this 50 times and you got it (it took me 25 minutes but that's just me figuring out how this works).

• There's actually a sniper case near this spot that you can use to replenish your ammo. The screenshots below are a recreation of the path from the sniping spot to the sniper case.

↑ Here i'm looking at the sniping spot from behind
↑ This one's just looking at the previous screenshot position
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I don't know if this was intentional or not, but it's a bonus.

• In the upper part of the road (the one were the sniper case is) there's a military checkpoint that has 5 guys roaming around. This is the spot of another method for this achievement, found out by scotty61990, or JudgeRhadamanthus (I don't know), where you can kill some more people with the sniper, but, this method requires you to travel far more back to respawn the guys, due to the ground blocking your view to the enemies, to then go back again and see them.
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• If you don't know how to zoom in the sniper, although it says it in the console version, by default it's Shift (increase zoom) and Control (decrease zoom).
  
  
• If the guy didn't respawn, it could be because you didn't go too far to despawn the pickup. Try to follow the road while going up and then going back once you have passed this wooden thingy with some trees.
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• Sometimes, another guy will spawn guarding the pickup. If you shoot the mounted gun guy first, he will run next to a rock and stay there without moving. If you shoot him first, the mounted gunner will do absolutely nothing. So it's a win/win choice.
  ↑ This is the "oh ♥♥♥♥ my comrade just died" position, next to the rock
  
  
• Even if the radio man says "HOSTILE SPOTTIDOOO! ATACC ATACC!", no backup will arrive to kill you, if you don't go over Heat level 1 (it goes up with each consecutive kill if you don't cool it off).

So... yeah. This is the guide. Also it's funny that the additional info is bigger than the guide itself. I don't mind likes so, you could just use this and do nothing, I would't complain, I do it too. This is the best way I have personally discovered, and I don't know if there's anything better. Feel free to let me know if there's a better way, or if I wrote something wrong, or how the ♥♥♥♥ can I get a space between an image and text below without having to put periods Here's the solution, use this "⠀".

Also if you wanna make gifs download ScreenToGif, it's like, just a .exe and you got sizeable gifs.

That's it. Thank you for using this guide. Have a great day, see ya.