Stun bullets are non-lethal ammunition assigned to Wulfenbach forces for use in urban or crowd-control situations ✣ where troops are deployed more as riot police than in a conventional military situation.
In our world, non-lethal ammunition for similar purposes rely on the transfer of kinetic energy to accomplish this incapacitation [1]. They consist primarily of rubber bullets, plastic bullets, and beanbags, are generally low velocity and are fired from shotguns, grenade launchers, paint ball guns, or specially-designed firearms and air gun devices [2].
An early non-lethal round consisted of short sections of broom handle fired at rioters in Singapore [3]. A "beanbag" round, consisting of a bag of shot, is designed to deliver a blow that will cause minimum long-term trauma and no penetration but will result in a muscle spasm or other reaction to briefly render a violent suspect immobile [4].
There are indications ✣ that Wulfenbach-issued stun bullets may be chemically based instead of merely depending on kinetic energy and muscle spasms. This would fit well with a military which uses C-gas and D-gas. However, even a poorly-aimed stun round ✣ has a potential to be lethal, as does any less-lethal ammunition, and Wulfenbach troops have been carefully trained in their correct use.
A primary use of stunning ammunition also appears to fit in with the Baron's philosophy of economy, i.e., not wasting resources. A live, bruised population is less resentful than one in mourning, and one never knows where one might find gifted soldiers, administrators or minions.