Here's the solution I saw used once:
All subskills for the Heavy Weapon skill are replaced as follows:
Heavy Weapons
-(Infantry)
Includes: ARMP, AT Gun, Field Mortar, Flamethrower, Grenade Launcher, LMG, LAG, RAM, Rocket Launcher
-(Cavalry)
Includes: AT Gun, Autocannon, FGMP, Mass Driver, PGMP, VRF Gauss Rifle
-(Field Artillery)
Includes: Field Mortar, Howitzer, MRL Pack, Mass Driver, Meson Accelerator
-(Armored Infantry)
Includes: FGMP, PGMP, VRF Gauss Rifle
Otherwise I'd just stick them under Man-Portable and rename it Direct Fire.