Great video!
I shoot all sorts of slugs outof my 870's 18" smooth cylinder bore. They shoot just fine (a couple of MOA). I don't understand why people are so surprised. I've got an EOTech on mine but it also has iron sights. The EOTech helps with accuracy at extended ranges (and helps with target acquisition speed, too).
He was shooting unsupported standing, so that he was all over the map is to be expected.
He was using open iron sights against a dark target.
Did you see the MAJOR flinch he has. It was most evident when he forgot to load a round in the chamber and dropped the hammer - HUGE FLINCH!
He shot a few high left and low right, hitting the edge of the barrel or the edge of the table top. The low right shooting is the result of the flinch.
That was a very decent demonstration of the accuracy you can expect with rifled slugs from a smooth bore shooting unsupported standing. They are much more accurate than he showed. I think he even acknowledged this. All around a cool video. Thanks to all.