The challenge you’ll need to complete in order to unlock the ZRG 20mm is this:
Using sniper rifles, earn 2 Longshot medals in 10 different matches.
It should go without saying that you’ll want to play
Black Ops Cold War to unlock the ZRG 20mm, if you own it. It’ll make your life so much easier. However, if you’re only in Warzone never fear - just run a sniper loadout and try for those sweet headshot kills. My best Kar98k loadout or best LW3 - Tundra loadout might be handy there.
If you’re struggling, check out these tips to help you unlock the ZRG 20mm faster in Black Ops Cold War and Warzone: