Ian McCollum has been posting videos of obscure firearms, and explaining how they work for a couple of years. He's also done video reviews on military and civilian firearm books, if that's more your speed.
Here's an example. Halloway Arms HAC-7 .308 Winchester semi-automatic rifle. There may be about 300 of these rifles. The design is a kind of a strange conglomeration of elements from the FN FAL, Kalashnikov rifle trigger and operating system, AR-15 bolt and gas tube. The rifle didn't work very well, and a piece had fallen off of it, but I think it's still an interesting design. I've been absorbing all that I can about military-style rifles for decades, and I've never seen or even heard of these Halloway Arms rifles.
Starting at the start, it seems that "Ian" is a bit of a gunsmith, engineer, and fabricator. I'm very jealous of the shop. Many many thousands of dollars worth of machinery, tools, and components there.
Hat-tip to Samuel Suggs for posting a link to the HK roller-delayed P9S pistol video.