Not outside the realm of the possible, but first we need to establish the responsibility of Baldwin, who was proximate. If the chain of custody was broken and another party snuck a live round it, how and why are pertinent but not exonerating of Baldwin..
Since I first replied to you I have been reading further, and i've seen people say that the weapons used in this show were old cap and ball type weapons, which would have been the earliest type of six shooters used.
I have never owned one of these, but i've known people who did, and if I recall properly, when you load them, you put the powder, wading and slug into the cylinder. I think the gun either breaks open, or the cylinder slides out, and you load the cylinder without ramrodding through the barrel.
Why this matters is if the gun was ever fired with a weak charge. A slug could have been pushed into the barrel and gotten stuck in there, but the next time it was fired with a proper powder charge, it would have dislodged the stuck slug and propelled it out of the barrel.
This may be what happened. This is what happened when Brandon Lee was killed with a firearm mishap on the movie set.