Squib load

A squib load, also known as a squib round, pop and no kick, or just a squib, is a firearms malfunction in which a fired projectile does not have enough force behind it to exit the barrel, and thus becomes stuck. This type of malfunction can be extremely dangerous, as failing to notice that the projectile has become stuck in the barrel may result in another round being fired directly into the obstructed barrel, resulting in a catastrophic failure of the weapon's structural integrity.

Causes
Squib rounds are possible in all black powder and smokeless powder based projectile weapons. They are most often caused by negligence in the powder loading process (insufficient or no powder load), or a failure of the primer to ignite the powder at all. While this occurs most often because of handloading by inexperienced reloaders, squib loads are known to occur even in reputable factory brands. Other causes include deformed bullets and attempting to fire a bullet which is slightly too large for the barrel, although both of these scenarios would more likely result in some variety of catastrophic failure instead of a squib.

Diagnosis
Signs that a squib round has occurred include: a much quieter or otherwise unusual-sounding discharge noise, lighter or nonexistent felt recoil force, discharge of smoke from the ejection port instead of the barrel, and a failure of the action to cycle (in semi-automatic firearms). That is why it is also referred to as "pop and no kick".