Although in this case, the suspect had worked for the Japan Maritime Self-Defense Force, so it wasn't just your average person off the street. At least I imagine that being in the navy would entail much more domain knowledge about firearms than your lay perosn.

Not very likely, military arms are so many steps removed from ‘improvised weapon’ that the only thing they share in common is they use burning propellant to shove a projectile out the end. Everything from chemistry, techniques, methods, materials is different.

Military is all about ‘at scale’ and ‘ready and waiting after decades’.

A couple pipes and end caps, a drill, some wire of different diameters, a 9v battery, and some common household chemicals and time and you’re good to go with what I saw the dude using.

Some odds you’d blow yourself up, but that’s easy to test out in the woods or whatever remotely.

All the above can be gleaned with some easy google searches or YouTube, including people making them and showing all the steps.

Not enough of a difference to make a difference. Single use shotguns from smooth pipes and black powder are very simple. Any lay person can figure it out.