"You shouldn't spread "knowledge" of a subject you know nothing about. The "end of it" is called a muzzle. It is also not necessary to cover up the muzzle as long as the barrel is angled down. The air inside will prevent too much water from getting in, and the downward angle allow water to drain. The US military's weapons, as with most of the first world's weapons, must pass rigorous durability tests before being accepted for issue. These tests being soaked in fresh and salt water, left in mud, and frozen then thawed. Every weapon is then tested for number of failures and mean rounds fired between failures. The Russian Ak-47 can be fire after being literally filled with mud. Ammunition doesn't need to be stored in waterproof containers as the casings are mostly water tight. If there was a gap large enough for water to get through easily, the gun may not fire, as the round would not be seated properly in the breech. If the round was seated properly, it could increase the chance of a squib shot (lessened pressure causing a bullet to become lodged in a barrel or preventing a semi/automatic action from cycling fully) or it could rupture the barrel if the round was able to be chambered. It would also decrease accuracy. In addition, spare magazines, clips, or loose rounds (however spares are carried for each weapon) are not stored in watertight containers as it is unnecessary and would impede access in a high-stress environment. An inserted magazine, a bolt cover, the bolt face itself, and the nature of a firearm's barrel all prevent mud and water from getting into the action. As long as a magazine is inserted in the firearm, the bolt/cover is closed, chances of mud and water becoming a problem are somewhat slim. Spare ammo is carried in pouches which do the same thing, to prevent mud from entering the magazine.