Judging from the size of his projectiles, the bore of his device is almost certainly over 12.7mm in "diameter" (it's square, not round, but it looks to be 0.5" by 1", and I'm not about to leave the hairsplitting to a federal bureau). Classification of a "destructive device" doesn't really care what type of propellant is used. I think it's pretty clear what he has here would be considered a "destructive device". It'll depend on his state whether or not it's legal for him to have.

https://en.wikipedia.org/wiki/Destructive_device