I agree there are affordable high grade weapons that can be made.
That brings us right around to one of the main arguments against gun control though.
Criminals don't obey laws. That runs right up to the edge of circular but it's true. If someone has decided to to accept the risks involved in commiting crime X, then crime x2, x3, x4, Y, and Z are basically free. They've already shifted from very low risk to very high risk and a little more won't stop them.
So... for people who are a threat... people who have decided to do harm... they've already decided to commit crime X. That means they've weighed the perceived outcome against the perceived punishment and decided that the risk of punishment is acceptable. If you've decided that the risk of life in prison, or execution, for murder is acceptable the rest of the alphabet, which means maybe a few months or years for possession of bomb making components or something, isn't such a big deal.
How realistic is that? Let's say they are a suicide bomber. If they are caught with bomb making supplies they'll face a few years in prison. If they are caught planning to blow up their bomb in a public place they'll get something longer. If they aren't caught they'll die.
How would that change if you got rid of the laws against making bombs?
If they are caught with bomb making supplies they'll walk, right? So farmer Smith with his supplies to make a bomb fit to blow up several federal buildings gets off.... unless he is caught loading those supplies into a rented van with a map to the nearest federal building of course. Then he'll get the longer sentence and his 'possession' is just evidence of the seriousness of his intent. He is punished. Justice is served.
OK, they don't walk... but maybe the guys who actually succeed in blowing up their makeshift bomb will walk... except if they weren't blown up with the bomb they've probably got a long list of capital murder charges or similar. So a few possession charges are probably not even going to be mentioned.
So what exactly does the law against these effective weapons do?
A few things. They increase the odds that non-outlaws will be punished for technical (vs. intentional) violations of the law. They reduce the liberty of people in areas where what you think of as WMDs are useful. I know people who have personally made large bombs... hundreds of pounds of the various ingredients. They did so legally (at the time) and (more or less) safely, on private property, for legitimate reasons (they wanted to dig a pond and that was more fun than renting earth moving equipment). They allow authorities to arrest potential bad people before they actually commit a major crime.
Hey... that last one seems pretty good, right? Except you take someone who was planning to commit a major crime, put him in prison for a few months/years so he can plan without having to work and isn't subject to normalizing social influences, you give him a chance to meet confederates, you give him one more reason to attack by branding him an 'outsider' and permenantly stripping him of civil rights and any real chance of positive reintegration into society as a productive member... and then you release him to go back and try again, this time with fewer penalties because you've already ruined his life and there isn't a lot more you can do to him except give him free room and board for a while.