I think you probably heard a weak report from the squib round and felt weak recoil, if you had stopped firing and inspected the weapon you could have just removed the bullet from the barrel and the American would have been fine.
A squib caused by a weak round will plug the barrel with the bullet, but you get the chance to STOP SHOOTING when you hear the weak report, and fix the situation.
If you ever get a round that doesn't seem to seat completely in the chamber, DON'T FIRE IT, it could be a double loaded round, where two bullets are seated in the case or a round where a single bullet has been seated too far into the case causing the case to expand slightly, and it could detonate, due to over pressuring, turning the gun into a hand grenade. I found one of these recently, a round with 2 bullets seated in the case, it was a commercial round from Federal, not a home grown reload. I weigh each and every one of my rounds now before loading them into mags.
If I had managed to seat that round, and fired it, I would be minus a gun and possibly a hand and/or an eye. It just goes to show, most accidents are inconceivable until they happen.