Massive Entertainment and Ubisoft decided to make The Division 2 feel less spongy in general and that translated to PvP as well. It opened up way for burst-damage-focused builds, such as the one-shot sniper as well as its counters.
The Division 2's one-shot sniper build is popular for several reasons. First up, it is not hard to grind the gear for the build so just about everyone can get it. Second, it allows players to instantly delete their opponents in PvP, and third - it is useful in both normalised and non-normalised PvP along with some instances of PvE. In short, you get a lot of bang for your buck.
To get it working, all the armour you need are three pieces of Airaldi Holdings gear that will provide 10 per cent accuracy and more importantly a 10 per cent increase to both headshot damage and marksman rifle damage. The other two pieces can be mixed or matched in any way you see fit, but two pieces of Gila Guard are usually a good idea if you are not planning on using a second primary weapon.
Speaking of, it is essential to have a Model 700 marksman rifle. Not the carbon version, the regular one. This weapon has the highest damage per bullet in the game and that is the reason why the headshot multiplier gets so wonky. It is not hard to come by as you can even craft the replica in Base of Operations and the blueprint will be available long before you are level 30.
It doesn't matter what talents you roll on the weapon, they might as well provide a bonus cookie when you are buying them from girl scouts. Mods are somewhat more important though. While x8 and x12 scopes offer chunky headshot bonuses, I would suggest avoiding them as they force you into first person and ruin your chances of fighting in mid and especially close range. You could equip two Model 700s though, one with the long range scope and one without.
Suggested specialisation is naturally the Sharpshooter as it offers an additional 25 per cent increase to headshot damage.
Mods on the gear are also important, however, so prioritise headshot damage and weapon damage on the red slots. That's it, no big secrets, it is just stacking damage modifiers on the hardest hitting marksman rifle in the game.
UPDATE: M700 was nerfed in a recent patch where its damage was cut by 13 per cent . Simply getting an M700 with Ranger talent should completely counteract this nerf. The build still works, just needs some actual work on it.
Countering it is another story though. Due to the one-shot potential, it is highly important to either impede an enemy's ability to shoot precisely or to shoot you at all. Firefly drone with Blinder mod can be used to ruin a marksman's life until their team shoots it down, giving you enough time to close the gap and face them in close quarters.
A more reliable counter is the Drone with Defender modification that will shrug off bullets so you will have enough time to close the gap. Sharpshooter specialisation is a decent counter as well, due to the flashbang grenade that can disrupt the one-shot opponent for long enough to run up to them.
As you may have noticed all of my suggestions on how to counter the build consist of closing the gap between you and the precision rifle lobotomy enthusiast. Therefore, the weapons that are suggested are the ones that excel in close quarters such as the Vector which has a massive burst of damage each clip, which should provide an edge over the marksman-rifle-wielding enemy.
Keep in mind that if you indeed have an SMG equipped, all specialisations will offer 20 per cent critical hit resistance in Conflict so plan accordingly. One of the best talent combos is Strained on the weapon and Berserk on the chest piece which will make you deal increasingly more damage as your armour gets worn down. It is excellent for close range encounters as you are nearly guaranteed to get hit.
On the other hand, you could optimise for shotgun damage and bumrush the sniper while wielding a Double Barrel Shotgun which will ruin anyone's day in close quarters. It also bypasses the critical hit resistance as it's more focused on brute force damage.