Generally speaking a "squib" load, at least in ctg. guns, is a load that's so underpowered that it won't even push the bullet out of the barrel. They are very dangerous because the bullet is still in the barrel and upon firing your second shot the bullet that's stuck in the barrel will act as a blockage and make the gun blow up.
Now, on the other hand, a "light load" in a muzzle loader is a load that pushes the bullet at min. velocity for use in killing small game or target shooting. I think a lot of people confuse the two terms and say "squib" when they really mean "light load".
[url]
http://www.ashlandlakegunclub.org/submachine_gun_clinic.html[/url]
"Re-loads
Absolutely a no-no. A squib load traveling far enough down the barrel will result in a burst barrel from the subsequent shot. This will result in damage to the firearm, and potentially fatal injury to the shooter. Remember, the firearm has no knowledge that a previous shot fire has not cleared the barrel. On full auto, you do not have ample reaction time to ?hold up? on a subsequent shot following a squib load."