The XM8 is a rifle that was originally designed to replace the M4/M4A1 carbines and M16 service rifles. It was intended to be the United States Army's standard issue firearm. However, the project was abandoned.
The XM8 was utilized by all of the Rat Patrol Team 01 members save for their leader, Meryl Silverburgh, who herself used the Desert Eagle. It was also utilized by the Army-Marine joint task force in Eastern Europe in an attempt to apprehend Liquid Ocelot, but they never got the chance to use the XM8s on Liquid due to his hijacking the Sons of the Patriots system, thus locking them out. It was also utilized by various PMCs.
Behind the scenes
In Metal Gear Solid 4: Guns of the Patriots, the soldiers that attack Liquid's boat on the Volta River are carrying XM8s. Rat Patrol Team 01 also uses these rifles. The XM8 has an integrated dot sight. It can be picked up right after the player meets Drebin for the second time. Head for the left path instead of the right one and find it underneath a collapsed mine shaft entrance. Enemies that carry an XM8 will be a Baseline Carbine version of it, while Rat Patrol Team 01 wield different versions of the weapon system, including the Sharpshooter and sub-compact variants. Johnny Sasaki is frequently seen using the XM8. Found underneath a shelter. It has an integrated red-dot sight by default, but has roughly the same stats as the M4 Custom and only one underbarrel attachment point for XM320, which puts it at a disadvantage against the M4 Custom in terms of flexibility and versatility