I‘m an FNG in the army rsv, been in for a month, dont take my word as fact
but, if you wanna use machine guns like the C6 an C9 an anti-armour, everyone is training on those in SQ, if you wanna use them a fair bit, join the Infantry, an if you wanna use anti-armour, request to take that couse after your MOC training, or if u want to be a machine gunner, request to take the machine gunner course
hope that helps