Cut shells are a good way to ruin a good or even cruddy shotgun.
A classic example of something one watches and realizes just how stupid and dangerous it is. The pressures involved, the steel involved, its much like driving across the country with bald tires that have wires sticking out. Sure people have gotten away with it, that doesn’t mean its smart.
As to shot making, taking two steel plates and rolling lead bits between them is easier and no heat involved.
One can make buckshot by taking spent .22 bulets and rolling them round with the plates.
birdshot can be made by cutting little cubes of lead out of a lead sheet and rolling them round again.
Of course one can always just use the “cubic shot” as is, and have a homemade copy of a cubic shot ‘spreader load”.
Shot towers use screens and a water bath after a long drop, the drop allows the shot to become round during the drop instead of the teardrop shape of the close quench.
Casting buckshot is time consuming, and a great deal of the effectiveness comes from what hardness lead you use and buffers/wadding. It does take a bit of experimentation. Still follow your shotshell reloading manuals as shotguns aren’t built to contain pressures that rifles are, nor are they tested the same way.