Forum

As written
HEAVY WEAPONS SPECIALTIES
- MACHINEGUNNER: Gives a +1 modifier to HEAVY WEAPONS rolls for firing light, medium, and heavy machineguns.
- LAUNCHER CREW: Gives a +1 modifier to HEAVY WEAPONS rolls for firing grenade launchers and missile launchers.
- REDLEG: Gives a +1 modifier to HEAVY WEAPONS rolls when firing artillery, including mortars, cannons and tank guns.

I will place in my suggestion first, and then explain why afterward.
HEAVY WEAPONS SPECIALTIES
- MACHINEGUNNER: Gives a +1 modifier to HEAVY WEAPONS rolls for firing light, medium, and heavy machineguns usually mounted on pintel mount or tripod.
- LAUNCHER CREW: Gives a +1 modifier to HEAVY WEAPONS rolls for single launch rocket, missile or shopulder fired grenades.
- REDLEG: Gives a +1 modifier to HEAVY WEAPONS rolls when firing indirect fire weapons such as artillery, including mortars, cannons and some grenade launchers.
- CREWGUNNER: Gives a +1 modifier to HEAVY WEAPONS rolls for firing weapons that are integral parts of a vehicles armament such as a tank gun or coaxial mounted Machinegun.

These weapons are all fired differently. Machine guns are typically fired form a Pintel mount or bipod/tripod but they are still a direct line of sight weapon. AGL's like the Mark-19 are fired pretty much the same way as a .50cal, so should fit into this category.
The second category were trying to include RPG's LAWs , TOWs, AT-3 Saggers and even stingers. These are shoulder-launched or launched form some sort of system that is still controlled manually by the launcher. I have fired LAWs, Dragons,AT-4 and TOWS. Each has it uniqueness and yes the TOW is guided to its target unlike the others, And dont get me started on the arcade style control of the Sagger, but they are still essentially tube launched missile systems. So grouping them al together here is fine for simplicity.
The redleg category was basically kept to indirect fire weapons , Artillery, Mortars, UBGL like the M203 and single shot GL like the M79, as well as Rocket artillery. Yes Huge diferewnce between firing a M203 (which I have fired) and a M777 which I have not, but again for simplicity i placed both here. Technically UBGL and handheld GLs could be placed into a category with shotguns or rifles as well as they are generally trained to fire a M203 along with the M16.
Finally the Crewgunner. THIS is your APC and Tank crew gunner firing from inside a vehicle through a vehicles sites and its interlinked fire system. Firing a weapon from a Turret is its own beast and regardless of the weapon be it a M242 Bushmaster 25mm chain-gun or its coax M249 7.72mm they are fired EXACTLY the same way. Even a Tow fire from the TUA system is fired differently than a TOW mounted on a Hummer. (which is fired the same way as its tripod mounted version)

As it stands a Gunner on a Bradley who would train with the 25mm chain gun, the coax MMG and the TOW missile as all part of his weapon qualification would have to gain 3 separate specialties to be considered Proficient with the weapon system. By placing them into a crewgunner [call it whatever) they gain the proficiency of firing the weapons system from inside a vehicle. As a future possible specialty you could add a remote turret operator to the specialties.

It's sometimes hard to draw the line between skills.

firing a LMG/MMG (HW skill) from a bipod is closer to firing an an assault rifle then firing an assault rifle is to firing a pistol (both rifle and pistol use Ranged Combat skill). Or a bow.

firing a M203/M79 looks pretty close to firing a rifle, but higher angle and slower projectile.

ATGMS are special. Usually slow (e.g. 10 seconds flight time to maximum range), which makes planning before firing harder. Will the target reach cover before the missile hits? At least they're not firing during two rounds now with rounds being 5-10 seconds. And even though I was trained on ATGM (RBS56, 2nd generation ATGM), hitting something with an 1st gen ATGM like AT-3 Sagger would be very difficult.

Indirect artillery fire without LOS is radically different from machine guns and atgms

But that's they way it is with RPG skills, a fine balance between too narrow and too broad.

CREWGUNNER makes sense.

Things like the M203 become square pegs in any round hole we try to stuff them into. Training to use the M203 won't help you fire Russian artillery piece; on the other hand, lumping them in with rifles means the kid who used to bullseye womp rats in his T16, back on the farm, can be expert on under-barrel grenade launchers too. Perhaps shoulder-fired grenade launchers should be a separate specialty, bearing in mind that this is not the skill to use the weapon, but a bonus to hit with it. Specialties can be specialized!

Machine Gunner ought to be a specialty, for medium and heavy MGs at least, and it ought to provide a bonus to score additional hits against additional targets in the same hex to reflect the correct employment of MGs as area weapons. Also Crewgunner should clearly be separate from Redleg.