In my case, I put the detonator mutator on my Silverback. Sometimes the mutator procs, and sometimes it doesn't. In particular, I noticed that it happens the most when I swap to my primary and then swap back to my sidearm. It stays busted until I reload.
#Detonator Mutator doesn't proc without reload
vestal gust
vestal gust
Actually, to be even MORE specific, it's bullets that are loaded by the Provisioner Ring do not get the Detonator mutator's buff.