To Protect the Me-109 pilot from bullets and projectiles, the fighter incorporated a bullet resistant windscreen, and steel and aluminum armored panels. A bullet striking an aircraft may pass thorough it’s air travel, multiple structures, systems, fuel tanks prior to reaching the cockpit armor. The bullet will have slowed down at armor contact. The bullet may start to tumble or yaw while passing through the aircraft systems. Both these attributes will reduce it’s penetration power. The German’s likely incorporated a large aluminum laminated aluminum bulkhead since it would be more resistive to a yawed bullet strike, than an armored steel panel.