Since you don’t really need a rifle much in this level, just select a bolt action one for your loadout before the level. There are Kar 98s throughout the level if you prefer to take a semi-auto and just swap out later, but be sure that you have any bolt-action rifle equipped before you call in the bombers. After the cutscene, run and gun until you can see the plane and keep shooting the engines with your bolt-action rifle. To date, this is the only secret achievement in the game.