Someone I know fired #4, 00 buckshot and birdshot through the weapon and it sounded like a grease gun or M2 .50 cal. He relates that it is very controlable and can be fired on full auto with one hand. This weapon cannot be sold to the public because its a full auto weapon and judged to be destructive by the BATF. The gun also has a counter recoill system.
You could have a semi-auto on a shotgun certificate but in the U.K. you would be limited to just 2 rounds in the magazine which rather loses the point of it. With an unrestricted magazine it would be classified as a "part one firearm", you would then need a firearms certificate to own one. Might just have a "bit of bother" in convincing your local Police that you had a "good reason" for it though!
This has been out for years, first designed by Maxwell Atchinsson (sp) back in the '70s who had an idea and a box of bits.
M16 forend & stock, M1928 pistol grip & a BAR M1918A2 trigger group !
They tried to market it but were beset by probs.
Later sold as the USAS 12, made in South Korea, they were pretty expensive and not a big hit Stateside after the AWB was enacted.
The first model ejected on the left for some reason, later they could be readily converted left or right.
This latest guise, the AA12, has a different operating system, and appears more similar to Atchisson's original commercial weapon.
I think it was just powder still burning in the case.
This is a common occurrence with repeating shotguns, it all depends on the ammunition.
I recall some years ago using some Raker rds which seemed to be loaded quite hot, and even pumps with bbls over 24" and fired relatively slowly were ejecting flaming cases !
In this film they appear to be Winchester Super-X, but there's only one box on the table and more than twenty-five rds were fired - other rds possibly ?