Bolt action
Despite being significantly slower to fire than lever-action rifles, which were invented around the same time, militaries across the world chose to replace their muzzleloader muskets with bolt action guns rather than lever action ones, because bolt action rifles are loaded over the barrel.  This allows easier loading by a shooter lying on the ground, without affecting aim.  With lever action guns, on the other hand, the ground is in the way of loading downwards, and to load the gun would mean to throw off the shooter's aim since the gun must be turned sideways.