Bullet pools are themed groups that deal a special effect. 

Some of the active pools in GunCrypt are: Lightning (chain and shock effects), Poison (slow, creeping damage over time), Vulnerable (armour shredding that makes everything hit harder), Root (crowd control for those who like their enemies standing still), and Ricochet (bouncing rounds that punish enemies for grouping up). Finding multiple bullets from the same pool is where the real magic happens.