Abnormal Ammo: While regular pistols, snipers and carbines (the three principal kinds of slugthrower) just fire flechettes chipped off an ammo block, a sluggun (or a military grade carbine that comes with an undermounted sluggun), while usually firing big slug-spikes of dense metal as an anti-materiel weapon, can instead fire flechette-canister shot (as a really heavy shotgun), bore-compatible grenades, or gyroc micromissiles.
These, in turn, can include exploding shells, incendiaries and napalm, cryoburn shells, nanoweapons (if someone’s set up a microwave power system for them), chemical/gas dispensers, cyberswarm dispensers, network node – or spy dust – dispensers, injector needles (at low power), restraint nanoglop, electroshock shells, acid globs, anti-electronic fiberdust, mollynet, antimatter nuke-in-a-bullets, and poisoned flechette-canister shot of various kinds, to name some of the ideas generations of devious arms designers have come up with – and also, for that matter, any random crap like sand or salt crystals you want to stuff in a canister.
Needlers fire, well, needles, but also fun things like the “crawler needle” that goes for a spin inside the body of its target and keeps hacking them up until even a glancing shot can be a kill. Ice needlers use a gravitic mass driver to fire ice needles, often with frozen drugs inside for remote doping.
This ignores, of course, the exotica, but then most of those are specialized weapons, not just specialized ammo.