For the record, in the mod they'd actually be mini-rockets that split off from one of the alternate rocket types as a sort of "cluster-rocket" concept.
Every weapon type has three to four potential alternate ammo types (not counting defaults) so far, though if I even make the mod, only two of them will be in there at launch.
Bullet weapons (Chaingun and Pistol) have Armour Piercing rounds (deal 1.75x damage against enemies, but has a lower firerate and the player can only carry up to 75 at a time), Ricocheting rounds (use custom Puff magic so that bullet impacts spawn another Hitscan shot that targets the nearest monster), Darts (do 0.1x damage per shot, but inflict poison), and Gyrojets (instead of a normal hitscan, the weapons fire projectiles at a lower firerate; the projectiles start out very slow and very weak, but gain speed and damage as they travel)
Shell weapons (Shotgun and Super Shotgun) have Incendiary shells (work in a similar way to the Legacy of Rust flamethrower), Slugs (fire a single bullet that does a lot of damage, but the player can only carry up to 12 at a time) and Flechette shells (do 0.5x damage, but each pellet has a chance of damaging enemies behind the target).
The Rocket Launcher has Homing rockets (fucking guess), Cluster missiles (already explained), Grenades (work like the Grenade Launcher from Quake I), and Shrapnel bombs (don't explode, but instead shatter into damage-on-contact metal shards, kinda like when you use the Rocket Launcher's altfire in Metadoom).
Cells are more complicated, so I'll describe the BFG and Plasma Rifle's respective alternate ammunition effects as separate bullet-points. Both have Cryo, Dark Matter, Solar and Argent cells.
Plasma Rifle stuff: Cryo cells turn it into a freeze-ray, Dark Matter cells reduce the firerate but vastly increase the damage, Solar cells slow the projectile speed but the projectiles can bounce off walls and the floors and explode after a few seconds, and Argent cells turn it into a beam-based weapon that deals low damage very rapidly and makes targeted enemies explode.
BFG stuff: Cryo cells make it fire a projectile with an AOE freeze effect, Dark Matter cells make it fire a projectile that works like the AR2's alt-fire from Half-Life 2 where it bounces around quickly and deals large amounts of damage, Solar cells turn it into a "murder-camera" that deals 1000 damage to every enemy within the player's field of view but turns the screen fully white for a few seconds after firing, and Argent cells make it function like the Doom 2016/Eternal version but uses half of the maximum amount of Argent cells per use.