I was thinking the same thing. There are definitely restrictions on the size/function of guns, and you have to fill out paperwork, get fingerprinted, etc to get past those restrictions.
To buy any gun at a store, you also have to have a background check, which you generally don’t have to do for knives.
Some say this one is fake. I’m not convinced either way. While this may not have been the “manifesto” that was found on him when he was caught, why couldn’t this have been written prior to the act and the one he was caught with written after?