5mm round

The 5mm round is a military grade ammunition type in Fallout, Fallout 2, Fallout 3, Fallout: New Vegas, Fallout 4, and Van Buren.

Background
A high velocity round designed for assault rifles, the intermediate 5mm M134 cartridge has been replaced by the more powerful 5.56mm round in assault rifles. However, it found a new life as linked, belt fed ammunition for man-portable miniguns on the battlefield, specifically the M134 cartridge and 7.62x51 millimeter rounds. It has also been retained as the ammunition of choice for paratrooper assault carbines, due to its light weight and portability. 

5mm round, full metal jacket


The standard 5mm M134 round is typically packaged in linked belts, designed for use in miniguns. The full metal jacket round offers excellent performance against soft targets, with or without body armor.

5mm round, armor piercing


A 5mm round crafted to defeat enemy body armor. The softer metal jacket covers a dense penetrator, while the bullet is loaded with a higher powder load, offering excellent performance against armor. However, the higher velocity reduces the amount of kinetic energy transferred to the target on impact, reducing damage somewhat.

5mm round, jacketed hollow point


A hollow point 5mm bullet, where the hard metal jacket is applied specifically to increase bullet strength and create a middle ground between damage potential and penetration.

5mm round, JSP


A jacketed soft point 5mm bullet, where the metal jacket is applied only partially to the bullet leaving the lead exposed to expand on impact. This allows the bullet to inflict more damage than a normal round while keeping some minor penetrating properties, however the downside is it causes stress on the weapon than normal.

5mm round, hollow point


5mm hollow point rounds feature a pit or a hollowed out shape at the tip of the bullet. As a result, the bullet expands upon impact, delivering more kinetic energy to the target and disrupting tissue more extensively. However, this same property also leads to dramatically reduced penetrative power, as the bullet will be stopped by armor more easily due to its properties.

5mm round, surplus


Although it might seem that ammunition remains generally the same, it is not so. Ammunition is constantly refined and new variants are introduced, replacing the old ones. The latter are either issued to second line units or sold on the market at discount prices. The 5.56mm surplus ammunition is exactly that, offering slightly improved damage at the expense of causing more wear to the firearm. It is available cheaply in bulk, however.

Behind the scenes
The variants listed on the side of in-game model for Fallout 3 and Fallout: New Vegas are actually variants of .50 BMG, not 5mm.