Each weapon type has a general role.
Pistol/SMG: Anti-light
Shotgun: Anti-Heavy/Light
Rocket: Anti-Heavy/Building, Sniper
Flamethrower: Anti-Light/Building
The Pistol has a damage and range bonus, and the basic shotgun gets the combat shotgun's damage bonus instead
Most ammo is built from previous tier of ammo, and generally uses coal as part of the recipe.
SMG ammo gets a bit of splash, and 30 ammo, instead of 10.
Shotgun ammo has only 4 shells.
Rockets have doubled range, of 45, outrangeing anything in vanilla. Standalone mod just has this change.
Rockets cost solid fuel in addition to their old cost.
Flamethrowers deal AOE damage, leave burning areas on the ground, penetrate enemies, and deal massive damage to buildings.
Gun Turret range is increased from 17 to 30 (Lasers are at 25)
Tank cannon range is higher and very effective vs buildings.
Grenade damage is increased significantly.
Bolter ammo for Pistol and SMG. Expensive, 10 ammo, deals explosive damage in AOE. Using with pistol makes it more cost effective.
Enemy structures are highly resistant to most forms of damage. Large explosions and Fire are the only things it doesn't resist.
Smaller zergs take increased fire damage. Larger zergs resist physical damage and take extra explosive damage.
Overall, you will tend to need multiple weapon types for effective combat, due to enemies tending to be highly resistant to some types of damage.