Coined by firearms journalist Dean Speir, kB! is shorthand for "kaBOOM!"
A situation where a firearm has a catastrophic failure causing it to explode. The situation typically arises from a ruptured bullet casing, but may also be caused by a barrel obstruction, or a having too much powder in a cartridge.
While a kB! will probably render the firearm inoperable, and unsafe to use, the firearm should contain much of the explosion.
I had my glock KB on me yesterday! The gun's shot, but I'm ok.