The M2A1 ammo can was adopted by the U.S. Armed Forces during the 1950s. Although it is often referred to as a .50 cal ammo can, the M2A1 is commonly used by the military and civilians alike for storing a wide range of small arms ammunition. The M2A1 ammo cans feature an integral rubber gasket. When its lid is fully closed, the gasket presses against the M2A1’s steel rim to render the can virtually waterproof. But here is the problem with the M2A1 design. If you stack several cans on top of one another or otherwise place a lot of weight on top of an M2A1, the pressure will gradually weaken the gasket and compromise its ability to seal out water. This isn’t a problem if you store M2A1 cans inside their original wooden crates. But without them, the M2A1’s gasket risks breaking when subjected to prolonged heavy pressure.

The newer M2A2 ammo can is identical to the M2A1 with one important exception: the M2A2 features an additional metal support bracket in its latch assembly. The bracket prevents the M2A2’s rubber gasket from getting crushed while it is under pressure (i.e. when multiple full cans are stacked on top of one another for extended periods of time). As such, the M2A2 protects its contents with a more reliable watertight seal.

Here are drawing images of M2A2 metal ammo can: