A rifleman has an M16 which can fire in single shot or burst mode, just like in the game. I think there are versions of the M16 which fire full automatic, but they are not usually issued to ordinary infantry soldiers because the ammo expenditure/hit ratio is very poor. In any case, the M16 is essentially a rifle not a machine gun.
An automatic rifleman has a SAW249. I don't know anything about this weapon except that it can fire on full automatic. It is a light machine gun, not an automatic rifle.
The difference (in general) between an auto rifle and an MG is that the MG can take belt feed; is usually equipped with some kind of tripod or stand (though some auto rifles have a bipod at the end of the barrel); you can change the barrel; the barrel is usually longer; and the chamber is designed to cope with a greater volume of rounds.
How to reflect this in OFP? Well, since its a squad based game not a platoon based game I tend to forget about platoon HQ .... the game doesn't have the units for it anyway. Carl Gustav soldiers are not very effective - 3 LAWs is usually better than a CG - and there is no SAW. So in OFP I would make a platoon up like this:-
HQ - whatever you want really, mission specific. Or don't bother
3 squads of 9 men. Each squad:-
1 officer
1 medic
2 machine gunners
2 grenadiers
1 or 2 LAW soldiers} or 3 dudes each with 1 (or 2) LAWs and grenades
1 or 2 soldiers }
Remember that the medic and the officer are like soldiers too.