In terms of customizing ammunition and such, nothing can compare to the versatility of the shotgun. The shotgun uses a shell, where the primer and load expels whatever is in that shell with high velocity. The most common is the standard shell that comes with multiple shotgun pellets, where the “spread” of the shotgun comes from.
For bigger animals, many use slugs, which is basically one big bullet loaded into a shell. The shotgun is so versatile because you can pretty much load anything in the shell that fits correctly, even if it may not be very effective. Because of this, there’s a population that like to produce custom ammo and make all kinds of wacky things. Occasionally, quite deadly rounds get produced.