So what is a squib load exactly? A squib load is where a cartridge is too weak to push the bullet out of the barrel of the gun, so the bullet gets stuck inside the barrel.
Here's what happens when a user encounters a squib load: The user pulls the trigger of the firearm, the hammer strikes the cartridge properly, but instead of a loud bang with the usual recoil, the firearm makes a softer "poof" sound and the recoil felt is much less. Instead of smoke coming out of the end of the barrel, it comes out of the ejection port.
So what causes squibs to happen? Often, this is because of poor quality control during cartridge loading. The person may not add enough propellant into the cartridge, or even forget to put any propellant into the cartridge. Sometimes the primer may explode without igniting the propellant. Also, the ammunition may not have been stored properly and the propellant inside could have degraded. These are problems usually seen with cartridges that are reloaded by inexperienced people, but they may also happen with factory loaded ammunition (especially if the manufacturer has a poor reputation for quality control). It may also happen that the bullet is misshapen or a bit too large for the barrel, which could also cause it to get stuck inside.
When this happens, it could lead to a very dangerous situation. If the user doesn't notice the squib cartridge and fires another cartridge with the barrel blocked, then there is a very good chance that the barrel could become bulged and the firearm ruined. There is also the possibility that the barrel could explode and lead to possible injury or death to the user and bystanders.
Therefore the user should pay careful attention to his firearm when shooting. If the firearm emits a quiet "pop" sound instead of a loud noise and if there's a lot less recoil felt, then a squib load should be assumed. The user should wait for a little while to make sure this isn't a case of hang fire and then unload and disassemble the firearm and remove the bullet stuck inside.
With all that said, here's what a squib load sounds like:
As you can see, the user encounters a squib at around 1:40 in the video. He wasn't paying much attention and thought it was just a misfire and was at the point of clearing his pistol to shoot another cartridge when the range safety officer yelled "stop" at him and pointed out that the previous cartridge was a squib load. You can hear the difference in sound when the squib fires.The user notes in his video that the cartridges were personally loaded by him and that he hadn't paid as much attention when reloading them and it is a very good thing that the range safety officer noticed what had happened and stopped him from shooting any more.