“That is, if all materials are produced in a state, and guns and ammo are sold only for in-state use, then eventually an argument can be made that the federal government has no regulatory authority over it.”
If the courts let them claim authority over grain grown on your land for your own consumption, then they’ll claim authority over anything they want.
You both raise the same point, that FDR’s corruption of the Interstate Commerce clause (as well as LBJ’s corruption of the General Welfare clause, have both been accepted by the SCOTUS, even though they should have been rejected on their face.
Thus, when conservatives are in power in congress and also have the POTUS, the Supremes should be encouraged to reverse these decisions and return the country to greater constitutionality.
But, in the meantime, the pressure being applied is by the 10th Amendment movement among the individual states, because eventually one or more of them will bring the lawsuit that is used to overturn the abuses.