I had a squib the other day while i was out shooting. i felt the difference in recoil and knew something was up so i broke the gun down real quick to make sure the round left the barrel which it did luckily.
This exact thing happened to me with my Enfield, I passed the rifle to my friend after firing half of my last magazine (5 shots) and when he went to fire, nothing happened, he pulled back the bolt and ejected what seemed to be an empty brass casing and he goes "Are you sure you chambered one for me?" having never had it happen before my friend chambered another round and was re-aiming at the target, I told him to hold up something wasn't right. I checked the shell that had fallen out and there were bits of gunpowder coming out of it, but they looked funny, charred but not ignited or something, idk how to describe it. I dropped the bolt out of the gun, pointed it up towards the sun and looked down the barrel, couldn't see any light, that is when I got that feeling where your heart jumps and you are like "Holy crap." Cuz I knew it would have been like a grenade if he had squeezed off that 2nd round. I had a Nagant with me and used the cleaning rod from that to gently knock out the round, kept shooting just fine, I kept the bullet as a reminder and a lucky charm, almost caused me and especially my friend some serious damage, possibly death.