Endgame, armour piercing rounds are only ever worth using in lots of turrets but due to the hassle of feeding them and their short range compared to laser, I don't bother. It is handy to have a stack in a submachine gun for when the biters are near your structures so you don't just destroy everything, but once you get walls up this isn't a problem.
Before you get your power armour the tank is okay. It's faster than walking and has excellent range with its cannon, but bases can be unassailable if near trees or water, and cannon ammo is very expensive and you burn through it very quickly. I only use this to get the artefacts necessary to build mk2 power armour and then it becomes a relic. Explosive cannon shells are a joke as they blow up on the first biter so you can't hit the bases.
The combat shotgun is currently the only weapon worth using, period, and once you get the mk3 power armour with a few shields you're just unstoppable. It's ludicrous that a single pellet of the shotgun does more damage than a shot of armour piercing rounds, and it shoots SIXTEEN of them per shot. Spawners go down in two shots and you can hit two at once.
Okay, to be fair the new flamethrower is pretty good too and is actually better at taking out hordes of big biters, but it's unwieldy, ammo is expensive and you're liable to burn down all your forests so some good pros and cons. Best paired with another weapon (combat shotgun).
Rockets are just pathetic, there is literally no reason to use them. They are very expensive to make, take up lots of room as you get one shot per item, range is only barely better than other handheld weapons, and at the end of the day you don't end up with any extra firepower. I actually just took some rockets to a spawner, one rocket and one explosive rocket didn't even take it below half health. (EDIT: Explosive rockets do even less damage than normal rockets! And it takes four rockets to kill a spawner.)
Here are my suggestions (numbers are assuming all research is completed, I know some of them have been suggested):
- Rockets need way more AoE, way more damage and way more range. Should be able to take out at least two spawners (see below) with 2 shots from well outside biter aggro range. While this may seem OP, keep in mind the cost in making them and that once you hit the base the biters will be after you anyway. Perhaps limit stack size to 50 to compensate. (Added) Suggest making rockets start at max damage, and the damage research is replaced with range and AoE boosts.
- (Edited) Spawners need to be harder to take down with guns. To keep the early game balanced this may need to be scaled with biter evolution. High physical and laser resistances, but no fire or explosive resistances. Grenades, rockets or cannon shells should almost be a requirement and take them down with ease.
- (Edited) Bullets need a higher clip size, perhaps affected by research, and shotgun shells maybe need to go down to 5 shots per clip (though considering the below this may need revision).
- (Edited) "Armour piercing" rounds should actually pierce armour, this would keep them relevant late but still balanced early. Turrets would be very strong late game but their short range offsets this. Armour piercing shotgun ammo should perhaps be replaced with shotgun slugs, very high damage skillshot that at max research just annihilates a few big biters in a line similar to the tank's cannon, especially effective against worms?
- Modular components in vehicles. Equip a fusion reactor (or perhaps it can burn the internal fuel?) and a few shields to increase the use you get out of your trusty old friend. Do you make an indestructable rhino or sacrifice some defense for lasers? You could even equip your car with a roboport for some better portable robots before you get mk2 armour, or permanently use it as a construction vehicle and have your own armour purely kitted out for combat. Such a simple addition (that doesn't even require any extra content) opens up a lot of decisions and customisation. Perhaps boost some stats such as roboport range.
- (Photoloss): Discharge defense needs tweaking for vehicles to become a powerful tool, mitigating the car/tank from becoming blocked in by biters to offset their low manoeuvrability.
- (Photoloss): Flamethrower revision, currently too powerful
- (Photoloss): Biter and nest behaviour revision (definitely agree with this but it's another topic altogether)