That first shot barely grazed his head, hence the hitmarker on the very tip of his helmet.  The second shot was lined up correctly and worked correctly.

You may want to watch the video again. The slow-mo part clearly shows about 70% of the spread on the enemy's head. On cqc, that should absolutely never happen. Change the Model for any weapon at that range and it's a guaranteed 2hk. The Model unfortunately was also a 2hk, which is bad for a shotgun.

The slowest full auto in the COD series...

If the USAS-12 was semi-auto with the same RPM, it wouldn't make a difference. It doesn't need full auto anyway. I wouldn't mind a USAS buff, its a 50-50 for me.

I watched it again, twice.  I can see that it's not lined up correctly.  I don't think that's a good example of the pump shotguns imbalance.  If that second shot resulted in a hitmarker, different story.

My video shows a better example.(I think) See Below.