Squib
A round that fires with insufficient power to push the bullet out of the bore — extremely dangerous.
Also: squib load
A squib load fires the primer (and possibly some powder) but generates so little pressure that the bullet lodges in the barrel. If the next round is fired, the barrel will rupture — a catastrophic and potentially injurious failure. After any unusually quiet shot or low recoil, stop, clear the firearm, and inspect the bore before continuing.