Here’s my thoughts on projectile weapons:
They don’t NEED to fill some unique niche role. They’re cool, they’d add variety. They can fill gaps in the fitting balance rather than battle balance. Heavy weapons with high crew requirements that need little power (mostly, rail guns should obviously need huge power). Another take being that projectile weapons are in fact stronger than their energy based counterparts, but suffer ammo shortages.
As for weapon classes, I see two predominant subcategories of these weapons; magnetic and chemical/conventional, each using their respective propellants. Magnetic weapons have prohibitively high fitting requirements, in crew, power and weight, but their range is longer than other weapons in their size category. They’re more accurate than plasma, and harder hitting. The downsides? Here’s an idea, aside from the fitting, the powerful but short lived magnetic field generated when firing causes damage to shield stability. Having too many of these weapons firing will drop your own ship’s shields temporarily.
Within this class I’m thinking of the following categories:
Railgun - large cannons comparable to plasma, but better all round (perhaps range ~1200). In view of the real life example people gave, perhaps these damage themselves slightly with every shot, meaning a repair module is necessary for continuous fire. Potential for weight classes within hull classes, ie. light and heavy railguns.
Gauss Gun - a shorter range, higher rate of fire weapon that doesn’t damage itself. Far less penetrating than the railgun, but higher dps overall. I’m thinking a range of ~600.
Chemical/conventional weapons are very heavy and have high crew requirements, but use very little power. Their range is less than average and they aren’t very accurate. However, their rate of fire and damage are both exceptionally high (penetration lacking though), making it the highest dps class there currently is, but short ranged and non-penetrating.
Within the class I’m thinking of:
Artillery - enormous great cannon, long range (~900?) terrible accuracy. Huge damage per shot and quite a respectable rate of fire for its size. This would be the end of any engine-less fleet, it would simply tear them apart. Cruiser exclusive.
Cannon - smaller class of cannon (perhaps hitting to ~700) with less damage, slightly higher rate of fire. Frigate size module exists.
Autocannon - rapid firing lower calibre weaponry, perhaps a range ~400, accuracy still pretty poor, but a huge rate of fire making up for it. Quite high damage, but sod all penetration. This weapon exists in all three ship classes, serving as the fighter’s rapid fire weapon too.
Flak - obviously enough, it’s a spray and pray anti-fighter weapon. Low damage, low penetration, after all it isn’t meant to hit them, just get near misses. Stupid rate of fire, it just puts up a wall of lead for fighters to run into.