Armor Piercing, Fin Stabilized, Discarding Sabot (APFSDS - yeah, long name), is an anti-tank round. I'm not taking cred, it was filmed, so you just had to frees at the right moment It's the heat from the "arrow" round.
The casing is ejected from the rear of the cannon. It falls down in a container inside the tank.
Ah, the good'ol SABOT round! I believe it's something like 720mmx19mm of depleted Uranium at 7,500 fps? I'd imagine there's few things in the world it can't penetrate given a decent impact angle. That's the American SABOT round, anyways. Not sure if it's a NATO standard or if different nations use different round types.
Must've been a really decent framerate, then. I wonder what framerate you'd need to see it not blurred in-flight? I think I'll go check youtube for some engineering videos of the SABOT
Found this. It's not a SABOT, but it is nice slow motion [link]
And this: This documentary clip is very basic (and shows some definitely-not-SABOT rounds hitting targets while talking about SABOTS, lol), but has some really nice slow-mo clips starting at 1:05 and again at 1:22. [link]
Thanks for the info on the casing. That was really perplexing, lol.
When I went to the Civil War reenactment nearby, a couple months ago, the cannons felt so powerful. They obviously weren't shooting anything real, but even at a considerable distance, my sternum felt the force every time! And those weren't even 120 mm guns either.