When Pvt. Barry Winchell reports to SSG Diaz he is told by Diaz that he will be assigned as the assistant gunner for the 40mm grenade launcher. This could be a reference to the Mk-19 40mm belt fed grenade launcher. However, we later see Pvt Winchell carrying an M-203 40mm grenade launcher. The M-203 is not a crew served weapon, it is operated by a single soldier and therefore doesn't need an assistant to operate it.