They used to function the same way as personal weapons, but it was changed a year or so back to prepare for facility-combat.
You still can use them against soft targets, but it's no longer advisable. They took a large damage nerf against anything other than facilities, Damage Types, and I thought their chance to hit took a big nerf as well, but I couldn't find mention of that on a quick glance.
Keep a stock if you wanna storm the castle someday, but if you're going to shoot creatures, droids, and sentients, stick to the personal weapons.
Correct me if I'm wrong -- and I have been once already this year -- but aren't Heavy Weapons usable against facilities on Derra and Serroco? I know they were last year, but haven't tested them this year yet.