This is the deadliest sniper rifle at this moment, being able to kill enemies with one shot at any part of the body unless there's a wall in between. The ZRG 20mm is the newest sniper rifle in the game it's main gimmick is being able to kill anyone at any range with only one bullet(Multiplayer Mode only). In BR mode it has high damage against vehicles and can down enemies with a single headshot as well making it one of the deadliest guns in the game. So here are the top 5 best attachments to use on the ZRG 20mm.  

MIP Light Barrel (Short)

This attachment improves the overall mobility of the gun which includes ADS speed, being able to one shot and move quickly is what makes a sniper rifle good. There are some downsides but they don't really affect the play style you want to go for as a sniper main. In-game description: Increases movement speed and flexibility at the cost of accuracy and range. Suitable for combat in a cramped space. Ups

• -7% ADS time - a must have stat improvement for this kind of sniper rifle, being able to aim fast and and kill with one shot is crucial.
• +5% movement speed - increased movement speed makes you have an easier time when moving with the gun
• +5% ADS movement speed - really good increase when you want to slowly peek while in ADS 

Downs

• +10% Vertical recoil - not really an important decrease as you want to switch into your melee after each shot
• +50% hit flinch - really affects how you'll play, you can't aim accurately while taking fire.

YKM Combat Stock

Further improves the ADS speed of the gun, making you have an easier time when quick scoping and playing aggressively. This also helps you aim faster against other sniper users, in those match ups the one with the faster aim usually wins the fight. In-game description: Slightly increases ADS'ing speed. Ups

• -10% ADS Time - faster aim makes you have a better time when using a sniper rifle.

Downs

• +30% hit flinch - another big increase in hit flinch, making this gun harder to use while you're taking fire.
• +12% Horizontal recoil - increased recoil
• +12% Vertical recoil - increased recoil
• +30% Aiming crosshair drift - makes it harder to aim for long periods of time.

3x Tactical Scope B

Now a lot of people might disagree with this attachment, quite honestly you can choose a lower zoom in the optics but this one is in a good spot for ranged and close quarter combat. Going for a higher zoom in the optics isn't really useful and will just slow your ADS speed, and since you're not really accurate when you take a hit it's not really worth it to take those. In-game description: Suitable for all distances. Ups

• Zoom

Downs

• Harder to aim accurately at longer distances

Stippled Grip Tape

Slightly improves aim speed at the cost of recoil and hit flinch, which aren't really important stats in this build. In-game description: Stippled grip tape maintains control when moving at a higher speed. Ups

• -5%ADS time - slight decrease on ADS time
• -15% Sprint-to-fire delay - allows you to aim faster after sprinting 

Downs

• +15% hit flinch - you really can't aim while taking fire with this gun.
• +4% horizontal recoil - not really an important stat
• +4% vertical recoil - same as before

Anti-Vehicle Mag

This is the core of this build and gun, the Anti-Vehicle mag gives a huge buff in your damage allowing you to one shot enemies even by just grazing them on their toes. A guaranteed kill on any part of the body, but this comes with a weakness which is lower penetration damage, so you won't be able to kill enemies behind a wall with one shot. In-game description: Fires large caliber ammo that significantly increases range and damage to enemies and vehicles. Ups

• +100% Damage range - increases the range on this gun
• +Damage - increases damage pretty much self-explanatory.
• +Body part damage multiplier - improves damage on other parts of the body.
• +300% Vehicle damage - increases damage done to vehicles (this is more noticeable in BR modes)

Downs

• -1 Magazine Capacity - one less bullet in the magazine 
• -10% ADS movement speed - makes this gun feel heavy when moving in ADS
• -4% Movement speed - slows you down while holding the gun.

  These are the top 5 best attachments to use for the ZRG 20mm. Some people might say it's better to get the laser than to get less zoom on the optics but honestly you'll have a faster aim time with less zoom and you won't even scope for long periods of time with this gun so you'll need to aim faster rather than zooming farther.