5mm round

5mm is a Fallout, Fallout 2 and Fallout 3 ammunition type.

Fallout and Fallout 2
In Fallout and Fallout 2, there are two types of 5mm ammunition, 5mm AP and 5mm JHP. 5mm was the standard US military round for bases located around the Core Region by 2077.

Fallout 3
The 5mm Round is a Big Gun-only ammunition in Fallout 3. 5mm is traditionally belt-fed, and is available in rather high quantities, something made necessary by the notably large "magazine" sizes on the weapons that use it--for example, the Minigun loads 240 rounds at one time and fires an exceptional rate, requiring that the player carries a huge supply if they intend to rely on miniguns.

Generally speaking, the 5mm Round is the most common big guns ammunition found in the game, as Super Mutants- especially Brutes and Masters- tend to favor the Minigun. It is quite easy to come by and highly effective later in the game when dealing with large crowds or Super Mutants, though robotic enemies take only marginal damage and Deathclaws seem to be less affected.

By spending time in various areas of Washington, DC, particularly the bunker area found near the Washington Monument, the player can accrue 5mm rounds quickly along with the .32 Caliber Rounds used by Super Mutants carrying Hunting Rifles. Members of The Brotherhood of Steel tend to carry miniguns as well, and with their alarming death rate outside Galaxy News Radio, it's possible to get some 5mm rounds here as well.

5mm rounds are made somewhat obsolete if the player finds a Gatling Laser. On the other hand, it is hard to keep repaired to full condition.

If the player has The Pitt add-on, 5mm rounds can be used easily to make other ammo types in the Ammo Press due to their abundance. Or, if in need of caps, the player can do the opposite, breaking down various other ammunition into huge quantities of 5mm ammo, each round of which has a base value of one cap. Considering that, for example, .308 Caliber Rounds have a base value of three caps, but can be converted into fifteen 5mm rounds each, you can substantially increase the total "value" of your ammo stockpile using this conversion.

Seeing that when the rounds are dropped, they are the largest of all ammo. So, if there are no barrels around to stack, these are the next best thing, just make some stairs with them.

Interestingly, Fawkes carries a single 5mm round in his inventory, but it is a unique version that cannot be picked up by the player or used by any weapon.