Pretty much title says it all. It just needs to not be afraid of explosions and pull a trigger or other activation mechanism on command, if it understands by itself when to shoot, then even better. Doesn't have to aim or move around the weapon, it is fixed.. It just needs to pull the trigger.
just my opinion (i dont know much about gun) assuming they use common gun trigger.
joke aside, snake can be trained or guide to at least, and i believe their constriction can help push the trigger, though i am not sure can they get used to the loud sound or vibration of the explosion or not.
otherwise i think monkey is a good choice, they can be trained even with audio command, and can hold or push the gun trigger at least, regarding not afraid of explosion i believe that they can be train to get used to the loud sound.
Insects are trainable, and since you're allowing "other activation mechanisms", I'd suggest that you could stimulus-response train an ant or tiny beetle to walk onto stimulus = YES triggering mechanism. The size of the gun so activated is up to you!
Just put the triggering mechanism under the little yellow ball and you can redefine the term "killer bee"!
Insects will serve your purpose well because their relatively simple nervous systems don't allow for higher emotions like "fear" and just from ordinary observation, insects don't seem to be particularly fazed by human made bangs like fireworks or gunshots.