Yep, you more that likely had a misfire. Nothing but the primer went off. Kinda sounds like one of those little white snap caps. You were smart to look for the slug in the barrel. Reminds me of the time when I was in the Marines. We were training with the shotgun, shooting slugs, and one of the shotguns had a misfire.The Marine ejected the shell and didn't say anything about it and simply handed the shotgun to the next Marine in line to shoot.That Marine loaded his rounds in and fired his first shot and immediately knew something was terribly wrong because the entire front half of the shotgun simply disappeared! He, as well as everyone else that was around him was very lucky not to get seriously injured. I was standing no more than 20 feet away from him when it went off. A little piece of "something" nicked my finger and the shooter's hand was numb for a day or so afterward, but other than that no one was hurt.

Nice job double checking that barrel!