It’s not a question of whether a specific cabinet department is explicitly provided for in the constitution (none are, but Art II section 2 provides for departments in general). It’s a question of whether the government was granted the power to do whatever the federal law attempts to do. I would say the USCG has two functions, a pure military function, and a law enforcement function.
As a military service the USCG contributes to the common defense of the United States. It operates as part of the Navy of the United States during wartime and special situations, and as a sort of reserve force otherwise. Article I section 8 grants Congress the power to raise taxes for the common defense, to provide and maintain a Navy, and to govern/make rules for naval forces. Article II section 2 designates the President as the commander of the Navy, and by extension, the USCG when operating as part of the Navy.
Unique among military services, the USCG also operates as federal law enforcement. Hence its organization under the Department of Homeland Security (in peacetime). In this role, as with all federal law enforcement agencies, the USCG derives its power to enforce federal law directly from the President. The President, in turn, is granted exclusive power to enforce laws by Art. II section 1, which vests the executive Power in a President (the executive power being the power to literally execute the law, therefore encompassing law enforcement). Article I section 8 grants Congress the explicit powers to, among other things, set (customs) duties, to regulate foreign commerce, to establish an uniform rule of naturalization, define and punish piracy and felonies committed on the high seas. Congress also has the power to make all laws necessary and proper for carrying into execution those powers, or even for carrying into execution the President’s executive power. I say the establishment of a department for enforcing federal law on the high seas and waters subject to U.S. jurisdiction falls within Congress’s powers.
My opinion is that the second amendment is a restriction on the federal government’s power to regulate the militia. The fear behind the amendment, at least as I interpret it, is that the federal government would disarm the states by (pursuant to Congress’s power to “provide for organizing, arming, and disciplining” the militia) mandating that no militia member may keep and bear instruments of war. The combined militias of the various states were intended to be a constant check on the federal government, specifically the President and his federal army. If Congress disarmed the various state militias, who would stop the federal government from turning tyrant? So the second amendment was supposed to close that loophole by preventing the federal government from disarming the people.
Today, the (constitutional) militia is defined as a) members of State Defense Forces, the National Guard, or the Naval Militia, and b) all able-bodied men between the ages of seventeen and forty-five. There are exemptions, notably all active duty armed forces. 10 U.S. Code § 246, 247. Not like I think the federal government gets to define the “militia of the several States”, but most states define their militias the same way. In a complete mockery of the second amendment’s intent, current law exploits another constitutional loophole and allows the federal government to literally draft all of the well-regulated militia members into the army, then ship them overseas for active duty. I think that’s technically constitutional.
But as a holdover from the second amendment, I would say banning militia-grade weapons (as a regulation of the militia) is unconstitutional. The idea is that the federal government can’t ban weapons that would be appropriate for militias to use… against a rogue federal government or invading force. As written, I say weapons of mass destruction (nukes, chemicals) could be banned by federal law, artillery and automatic weapons could not.
Note that I said federal bans. I also think the states get to decide who goes in their militias. I also don’t “incorporate” the bill of rights against the states. If a state wants to disarm its own populace, or disband its own militia, or only allow military grade weapons for actual national guard units as part of a legitimately well-regulated militia, one that meets Congress’s standards, that’s perfectly constitutional in my eyes. I personally think this latter option is the ideal one.
In practice weapons bans are proposed as exercising power to regulate commerce among the several states, not the power to regulate the militia. I would not go so far as to extend this power to intra-state transactions or manufacture (even narrower than United States v. Lopez, 1995; such that the Federal Assault Weapons Ban would not survive a court challenge), but that’s my opinion.
I was speaking of the federal government as a whole, not the judiciary in particular. If we’re getting into particulars, I had Congress in mind. The Constitution prohibits states from entering into compacts with each other unless Congress provides permission. Congress has plenary power over interstate commerce, including navigation of waterways. Congress has plenary power over printing of money and naturalization law. &etc.