[Alpo]

A squib is an intentionally underpowered load.

A 38 Target Wadcutter is a squib.
An Aquila Colibri is a squib.
"Cowboy" loads are squibs.
If you took a 308 and loaded it with a 150 grain cast lead bullet at 1100 fps, it would be a squib.

A bullet with no powder in it, that does not make it out of the barrel, is a dud.

For some reason, people, in the last few years, have started calling duds "squibs". They are wrong. There are many people using the term. They are ALL wrong.

So, whenever anyone tells you that a bullet getting stuck in the barrel is a squib, laugh in their face, because they are wrong.

When you have a bullet stuck in the barrel, and you fire another round, the new round goes charging up the barrel, pushing air in front of it. Since the barrel is plugged (and how it is plugged does not matter - whether it is a lost cleaning patch, or you stabbed the muzzle in the mud, or you have a stuck bullet) the air in front of the bullet cannot escape, so it starts to compress.

As it compresses, the pressure gets higher. One of three things is going to happen.

The increased air pressure blows the plug loose, without damaging your gun. This is the best of the three. You Lucked Out.

The pressure gets so high that the barrel expands, and then the plug gets blown loose. This results in a "bulged barrel". Sometimes the damage is very slight, and you cannot see it from the outside, but you inspect the rifling, you can see a circle in the rifling where the barrel started to stretch. This is called a "ringed barrel". Other times the damage is great enough that you can see and feel the damage from the outside. Your gun is not dangerous now, but because of the bulge it may no longer be accurate.

The third possibility is that before the air pressure blows the plug loose, the barrel wall ruptures. This is a "burst barrel".