First off, anybody that calls his video channel "PewPew Nation" tells me I'm not interested.
I agree that #3 did not only sound hotter, but also had a larger muzzle flash.
Shot 5, the squib, had a whisp of smoke that appeared round the cylinder. So although I did not hear a primer pop, just a click, something definitely fired and exited the case. Shot 6 was also different. Not quite as hot sounding as #3, but hotter than #4. It's anybody's guess if the slug from #5 exited the barrel.
But I agree wholeheartedly with Tom. He should have stopped right there and check for a barrel obstruction before moving on. Even if he did see something exit the barrel, it was an excellent opportunity to demonstrate proper safety procedures.
A late friend of mine (who owned Merit Gun Sight) told me about taking his mom out to the range one day. She was firing his K-38 when she had a squib. Before Cliff could say anything, she pulled the trigger again and the gun went bang. So, he let it go. When he got home and was cleaning the gun, as he ran his fingers down the barrel with an oil rag he felt a bump. Sure enough, about half way down the barrel was a tiny bulge. He said it never seemed to affect accuracy.