Included as part of the weapons for the Medic class in Battlefield: Bad Company 2 Vietnam was this weapon:
The weapon’s designation (XM22) and its configuration are at issue. The weapon depicted is supposed to represent a member of the Stoner 63 family produced by the Cadillac Gage company. This weapon was intended to be a modular weapon “system,” capable of being quickly configured as a number of different types of weapons.
A number of configurations were tested by the US Army during the early 1960s, and a limited field test of a number of configurations was conducted in Vietnam by the US Marine Corps in 1967. The US Army applied the designations XM22, XM23, and XM207 to the rifle, carbine, and light machine gun configurations respectively. The first two were possibly both in the rifle category (the full designation being Rifle, 5.56mm, XM22 and XM23), while the latter was in the machine gun category (Machine Gun, 5.56mm, XM207)
The weapon depicted is definitely not the XM22 rifle, but it interestingly enough isn’t the XM207 machine gun either. The US Navy SEALs, by far the most prolific user of the system, applied the designation Machine Gun, 5.56mm, Mk 23 Mod 0 to a variant that combined various features from different variants. The most notable features of this configuration, generally referred to as the “Commando,” were the belt feed and a carbine length fluted heavy barrel. The SEALs used the Mk 23 Mod 0 through Vietnam and into the 1980s, with the last operational use supposedly coming during Operation Urgent Fury in 1983.
The weapon depicted in Battlefield: Bad Company 2 Vietnam is most visually reminiscent of the light machine gun configuration, with its belt feed and full length barrel. However, it has the left-hand feed of the Commando. The switch of the side for the feed came after it was found in the light machine gun configuration that ejected cartridges bounced backed into the ejection port (referred to as “spin-back”), causing jams. The switch necessitated moving the charging handle, which then became inaccessible, to the underside of the weapon’s forearm. This is clearly missing too.
The Stoner 63 family has what could best be described as a cult following in many circles, from firearms enthusiasts to those interested in the history of the Vietnam conflict. There is really no shortage of images, if actual information is a bit sparse. However, the entry on Mongo’s Machine Gun Pages for the weapon (found here) has been around for some time and clearly points out the differences in the variations (if getting the designations wrong). The designations are clearly explained elsewhere on the web, including in the lazy man’s source, Wikipedia. It seems like the research done on this was pretty lazy too.