5mm round

5mm is a Fallout, Fallout 2 and Fallout 3 ammunition type. It was also scheduled to appear in Van Buren.

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.

Used in

 * Minigun
 * Eugene

Locations
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. 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. 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.

Van Buren
5mm rounds are still in use by NCR in 2253, and were used in all of NCR's standard-issue assault rifles. All of the NCR goons in Hoover Dam and Ballistic Orbital Missile Base 001 carry a bunch of spare magazines.