Reach rank eight for any of the four Armor classes to unlock one of the Pesticide guns as a reward for reaching the maximum rank.

Pesticide	Rank armor
Pesticide Cannon (Tier 8 Rocket Launcher)	Trooper Armor.
Pesticide Missile (Tier 8 Missile Launcher)	Tactical Armor.
Pesticide Mortar (Tier 8 Grenade Launcher)	Battle Armor.
Pesticide Rifle (Tier 8 Sniper Rifle)	Jet Armor.

Note: You can only reach Rank 8 by playing under the Inferno difficulty setting.