| ||For an overview of 5mm round in the Fallout series of games, see 5mm round.|
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.
5mm rounds are made somewhat obsolete if the player finds a Gatling laser. On the other hand, these are hard to keep repaired to full condition, making 5mm weapons a good backup weapon.
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, D.C., particularly the bunker area found near the Washington Monument, the player can acquire 5mm rounds quickly. Members of the Brotherhood of Steel tend to carry miniguns as well, and with their alarming death rate outside GNR building plaza, it's possible to get some 5mm rounds here as well.
Lucky Harith also carries this ammunition (up to 1000 rounds) when fully invested in.
When dropped, 5mm rounds have the largest containers.