The current ruleset states that:
A Heavy Weapon Specialist (HWS) is any Grenadier or Rocket (Simulated) carrying player. They may carry M-203 GL, M-320 GL, M-79 GL, GP-25 GL, M-136 AT4, M-72 LAW, RPG-7, Mortar or Arty Piece.
My suggestion is that this rule excludes the M203, GP-25, and any other underslung, single-shot grenade launchers (with no exception for standalone, single-shot GLs). My reasoning behind this suggestion concerns both realism and gameplay balance.
Firstly, from a realism standpoint, a rifleman with an M203 would not replace a squad-member with an M32 or AT4. The M203 is meant to be used as indirect fire against soft targets. With this in mind, the M203 doesn't seem to fit the intended purpose of the HWS, which is confined to an anti-vehicle role.
As for gameplay balance, it doesn't seem fair that a rifleman who wants to carry an M203 and only a couple of rounds would take the HWS slot from someone with an AT4 and 12 rockets. Considering that many players with the HWS role use shoulder-fired, pneumatic (as opposed to CO2/green gas) rocket launchers, it doesn't seem like an M203 should fall into the same classification of firepower and combat role. Non-HWS roles are already allowed to carry hand grenades, and I believe that an M203 is a lot closer to a hand grenade than a shoulder-fired launcher.
I would like to see riflemen be able to use underslung grenade launchers in addition to (or perhaps even instead of?) the hand grenades that they're already allowed to carry. A good compromise would be to only allow them to carry 2-4 max 40mm grenades at one time. That way, players can bring their M203s/GP-25s without losing the potential of a dedicated anti-armor specialist.