Age of Empires Series Wiki

Infantry equipped with firearms gain more hitpoints.
—In-game description

Flint Lock is a technology in Age of Empires III that can be researched by European civilizations at the Arsenal once the "Advanced Arsenal" Home City Card has been sent (except for the Swedes); the Japanese can also research it at the Golden Pavilion wonder. Once researched, it increases the hit points of gunpowder infantry by 10%.

Also, when revolting into Haiti, it increase Pirate hit points by 10% if the "Letter of Marque" Home City Card is sent.

Arsenal technologies
InfantryInfantry Breastplate · Counter Infantry Rifling · Socket Bayonet · Flint LockA · Paper CartridgeA · Military DrummersA
CavalryCavalry Cuirass · Ranged Cavalry Caracole · PillageA
ArtilleryGunner's Quadrant · Heated Shot · Professional GunnersA · TrunionA · Incendiary GrenadesG
A requires the Advanced Arsenal Home City Card
G only available to civilizations with Grenadiers