Back to the topic at hand, why not just restrict the additional weapons that can be carried with the HK69 with the addition of each round. Let's not forget that bulk does not necessarily have to mean weight, but, also size.
Basic weapon interaction rules should balance the loadouts without getting too complex.
One way this could be accomplished if you wanted to play as a grenadier for the round would be to include a bandolier in the loadout that would restrict the use of other weapons, either through code or being prohibitively expensive. The player could have access to two 10 round bandoliers and he/she/it would be restricted to carrying only pistols with a few mags of ammo.
The M203, on the other hand could be restricted to one 10 round bandolier and two mags of ammo for the M16A2 with only a pistol and two mags as backup.
To prevent too many players from playing the role of squad grenadier, a server side setting could force a new loadout for any players over a predetermined cap based on the number of players on each squad.
Any bulk <i>price</i> payed by the player's stamina should be alleviated with every round they volley. In other words, it should be easier for them to move around with the loss of bulk when they fire.
EDIT: Oopsie...I meant HK69