Mr. SHERMAN, was for securing the rights of the people where requisite. The State Declarations of Rights are not repealed by this Constitution; and being in force are sufficient.
https://avalon.law.yale.edu/18th_century/debates_912.asp
See also the 10th amendment. This is a jurisdictional issue. Every state, thankfully, has its own Bill of Rights.
We do not have "one big nation", the Founders repeatedly warned against that sort of dangerous scenario. - commonly, when referring to Montesquieu who was one of the important intellects behind the idea that big republics are big failures. We do not want "one big nation", we want the states to have powers and rights to use against the federal government. We want our localities empowered.
Let's take for example Texas, their constitution states:
THE TEXAS CONSTITUTIONARTICLE 1. BILL OF RIGHTS
Sec. 23. RIGHT TO KEEP AND BEAR ARMS. Every citizen shall have the right to keep and bear arms in the lawful defense of himself or the State; but the Legislature shall have power, by law, to regulate the wearing of arms, with a view to prevent crime.
This would, reasonably, allow for passing laws to (legally) disallow pre-teens from owning or purchasing firearms.
Iowa is perhaps even more interesting, as they are currently working to advance a "second amendment" for the Iowa State Constitution where they currently do not have one. They need one.
The states are generally more conservative than Washington D.C., so I will always side with the states having this kind of power over granting it to the Federal government.