Oh? It seems to me that Congress has the power to make all our Federal laws: under Article 1 Section 1:
"All legislative Powers herein granted shall be vested in a Congress of the United States, which shall consist of a Senate and House of Representatives."
More definitely, Congress shall not only make laws regarding providing for the general Welfare, but also promoting the general welfare: under Article 1 Section 8:
"To make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof."
That is, this portion empowers Congress to provide for (funding the execution of laws regarding) the general Welfare, and to make laws promoting the general Welfare, which is the objective of the Constitution as stated in the Preamble:
"We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America."
Prohibiting minors, felons, and mentally unstable persons from buying, selling, transporting, owning, or possessing firearms and ammunition would seem to me as intended to promote the general Welfare, through Article 1, Section 8 of the Constitution.
Why is this not an enumerated power of Congress, in your thinking? The laws passed will be enforced by the Executive branch, who can organize the enforcement, and formulate concomitant regulations, as it sees fit; and all being overseen by the Judicial branch.
What is the problem?
Article 1 Sec 8 lays out the specific powers of the FedGov. Making any asinine law at all under the "general welfare" clause does not hold up under even casual scrutiny. Especially since the BoR adds further "declaratory and restrictive" clauses.
Clauses like "shall not be infringed". In no guise can "promoting the general welfare" be seen to include promoting gun control.
If your idea of "promoting the general welfare" includes setting up a US Department of Santiy to determine who can exercise their Rights and who can't.... Then we obviously have very different ideas of what "freedom" entails...
The words "necessary and proper" apply only to "powers" vested by the Constitution or among the "foregoing Powers".
The preamble to the Constitution explains WHY the Constitution was implemented but the reasons listed do not amount to vested powers. Nowhere is there a vested power which amounts to "Congress shall have the power to regulated the keeping and bearing of arms by the people". In fact, the Second Amendment says just the opposite.
The words, "Promote the general welfare", contain no limitation whatever on what might be considered by Congress to be consistent with this phrase. There would be no need for any enumeration of powers if this was what was intended by our Founders.