Considering the sorts of mayhem already available out there, you seem to be going about things the hard way.
For small arms there are armour piercing rounds, incendiary rounds, hollow points, "safety slugs" designed to rapidly expand and dump all their energy into the target, 4.7mm rounds with asymmetric noses to cause them to rapidly tumble in the target, 12 gauge grenade rounds for shotguns, 25mm grenades which can be programmed to airburst over the target.....
Since these bad guys are "diabolical", they will probably get more bang for the buck by choosing "where" they shoot their targets rather than what they shoot them with. Being gut shot or kneecapped will cause a great deal of pain and suffering. Of course that requires very careful shot placement on the part of the shooter. The general rule in a gunfight is to aim for the centre of visible mass (to maximize the probability of getting a hit), and to keep firing until the target is down (so they are not shooting at you).
If it is just going to be a gunfight, then the ammunition chosen will have to reflect the expected target type. Shooting safety slugs at a person wearing body armour is not going to be very effective, but armour piercing rounds from a rifle can go "through and through" an unarmored human target without immediately stopping them (unless you hit a bone or vital organ).
Without knowing more than Quentin Tarantino-esquea setup, my recommendation is the bad guys should use sawed off 12 gauge shotguns loaded with 00 magnum shot. This provides for pretty violent gunfights at short range, the ability to deliver gruesome damage to human targets and satisfies the requirements for a successful gunfight.
As far as warfare is concerned, there is already far more than enough "stuff" out there to ensure gruesome outcomes for everyone.