Squib
Also: Squib load
A cartridge that fires the primer but generates insufficient pressure to push the bullet out of the barrel, leaving a dangerous bore obstruction.
Squibs result from missing or critically light powder charges. The primer pop alone is enough to lodge the bullet partway down the bore. Pulling the trigger again on a squibbed barrel forces the next bullet into the obstruction, generating extreme pressure and potentially destroying the rifle and the shooter.
If a shot sounds weak or feels dramatically different, STOP and do not fire again. Open the action and remove the magazine. Then push a rod down the bore to verify the barrel is clear before loading another round. Squibs are rare in factory ammo and show up more often in handloads with inconsistent powder metering.