In every firearms class that I find myself in, before we issue out brass and get ready go loud, a number of subjects are covered such as range alibi fire, first responder etiquette, eye and ear protection, basic safety guidelines, and what to do with hang fires, misfires and squibs. Squibs? Someone nearly always asks? What's that?
Glad you asked.
What it is...
The point by point definition of a squib load is, according to the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI), Glossary of Industry Terms, L,: "A cartridge or shell which produces projectile velocity and sound substantially lower than normal. May result in projectile and/or wads remaining in the bore."
To make that even simpler, it's a cartridge without enough umpf to push the bullet. This is bad because if that projectile does not clear the muzzle, and you fire another one right after it, you could have a kaboom
These are caused when either not enough propellant is used, a primer is insufficient to ignite the propellant, a bullet is seated too deeply into the case or is too large for the case neck, or any combination of the above is encountered.
What to look for
That's just it, its almost impossible to tell just from looking at an unfired cartridge, whether it is going to be an underpowered squib load or not. About the only time you will be able to do so is in a case of a misloaded round.
These two cartridges are both 9x19mm parabellum/luger. Both are 115-grain FMJ bullets on a brass case with a small pistol primer. Can you tell which one is seated incorrectly and probably should not be fired? Remember, if you have a box of ammo and one of these things does not look like the other, you may want to give it a wide berth.
Another squib maker is the dirt cheap Swedish 9mm KPTR M/39 gallery practice ammo that has been floating around for about a decade. Designed for the M45 and M45/B submachine gun when fitted with a special barrel attachment, its not meant for use in pistols.
This stuff is very distinctive with its black bullet, silver tip, and a blue band around both the bullet and the primer. It has a plastic projectile with a 5mm steel BB in the tip. The thing is, its been reported that this stuff won't cycle standard semi-automatic pistols (like your XD), and worse, the rounds often lodge in the barrel, making a squib load jam. If you have some of this stuff and just want to shoot it, check that barrel for blockages after every round, just in case.
What to listen for
When firing, should you hear a round that sounds 'off', for instance when you go "pop-pop-pop-pip," immediately stop firing. That's because you may have just experienced a squib load. It will often be accompanied by a hiss and a puff of smoke coming from the breech area of the pistol. Unload the firearm keeping your finger out of the trigger well and the muzzle oriented to a safe direction, then when safely possible, field strip it and check the barrel for obstructions.
For a graphic view of what this sounds like, here is a great video from Gunjock Shooting Sports that experienced a squib load while firing his XD45 in an IDPA Action Pistol Match in 2012. Watch for the smoke; listen for the hiss and not the 'pop.'
And there you have it boys and gals, the basics of a squib load, and how to avoid that all-too dangerous kaboom on the range. The fingers you save could be your own.