Heavy weapons are cool but not very suitable as your main weapons. More often than not they’re situational but when the situation rises, heavy weapons can make all the difference.
Heavy weapons deal massive damage, some even AOE damage, but they also have a major downside: you can’t run or fly with a Heavy Weapon equipped. So they’re great to initiate a fight by dealing massive damage but as soon as you need to be mobile because the opponent is shooting at you or getting close to you, you’ll need to switch to a non-heavy weapon.
Note that Grenade Launchers are not heavy weapons!
Ammo = Slugthrower Ammo
The best of the heavy weapons. Reliable and stuns enemies. Buy all Slugthrower Ammo you come across. Or craft it, but then you’d better download the mod from Nexus that balances the cost for crafting ammo and the return you get from crafting.
Ammo = Rocket
Rocket launchers are fine. Their ‘raw’ damage is higher than slugthrowers but 1° Their capacity is lower so you have to reload more often; 2° Their fire rate is lower. So while rocket launchers deal more damage, slugthrowers deal more damage per second. Rocket launchers do have a great AOE effect and the rockets stun whatever they hit.
Ammo = Spear
Spearguns are very good. You can only load one spear at a time but reload time is short (similar to bows). They have a nice stun effect but of course no AOE.
Ammo = Cannon Ammunition
What the…? When firing a flak cannon it looks like a few sparks are coming out of the barrel. So you don’t see a projectile but the enemies do get damage. Flak cannons have no stun effect. So all in all, stay away from these underwhelming weapons.
Ammo = Plasma Cell
Plasma cannons are awful so avoid them like the plague! The plasma hardly ever hits the enemy and when it does, it has no stun effect.