When I turned the cylinder by cocking it again from single action, I think the squid fell back into the cylinder on the next round. I felt like something was weird because I got such low velocity on the chronograph, so I lowered the hammer. I did not realize what had happened until I tried using a rubber mallet on the cylinder because I assumed the ejector pin had come loose again. By hitting it with the mallet I must have stuck the squib more firmly in place because for whatever reason the hammer will not cock again. I can however see the squib between the forcing cone and the cylinder, and of course looking down the barrel.
OK, let's see if I got this straight--you were shooting your revolver, presumably off a bench and chronographing your shots.
One shot recorded a low velocity, but you had already cocked the gun for the next shot, so you lowered the hammer on a live round and that's when the bullet (presumably stuck in the bore/forcing cone) "fell back" into the chamber mouth and got wedged in tight when you tried to open the cylinder with a mallet--right?
If your chronograph recorded a velocity for the last shot fired, something had to have exited the barrel, no? Perhaps because of the low velocity loading the bullet got stuck in the barrel, the lead core separated from the jacket and went out the barrel, thus giving you a velocity reading. The jacket would be pretty well lodged in the bore, so how could it "fall back" into the chamber on the next shot?
When you tried the lag screw did you get pieces of lead out of the stuck bullet? In other words, do you know if the lead core separated from the jacket?
If the jacket got stuck in the bore I don't see how it could shrink enough to fall free of the rifling and drop back into the chamber in the short period of time between firing the shot and cocking, then lowering the hammer (unless maybe it was very cold at your shooting point? Could cold weather have caused a jacket to "shrink up?" IDK nothing about cold weather--I'm in Honolulu.)
Was your bullet a Speer half-jacket by chance? I've read warnings about not using those particular bullets with reduced loads specifically because of the risk of core separation. For that matter, were you even shooting reloads or were they factory?