Here is the full list of all 21 GUN KING achievements.
Kill your first pawn.
Survive the full 20 minutes without getting hit.
Survive the full 20 minutes with any gun.
Survive the full 20 minutes with the Pistol.
Survive the full 20 minutes with the Shotgun.
Survive the full 20 minutes with the Crossbow.
Survive the full 20 minutes with the Machinegun.
Survive the full 20 minutes with the SMG.
Survive the full 20 minutes with the Burst Carbine.
Survive the full 20 minutes with the Grenade Launcher.
Survive the full 20 minutes with the Flame Thrower.
Survive the full 20 minutes with the Chain Lightning Gun.
Survive the full 20 minutes with the RPG.
Survive the full 20 minutes with the Freeze Machinegun.
Survive the full 20 minutes with the Laser Gun.
Survive the full 20 minutes with the Flare Gun.
Survive the full 20 minutes with the Turret Gun.
Survive the full 20 minutes with the Flame Turret.
Survive with only 1 HP.
Survive the full 20 minutes with Proximity Mines.
Unlock all guns.