In the terms of the day the Regulars were those that had their Arms provided for them and Militia classically brought their own. Regulated and Regular are related in this context and it relates to provision.
This is not a declaratory clause relating to an enumerated power (the specific enumerated Powers for Congress to govern the formation, calling out and training of the Militia were given in A1:S8) but is a justifying statement that explains the right subsequently recognized.
A Militiaman was expected to provide their own Arms suitable for service therefore the Federal was forbidden to attempt them to from trying to do so. That logically should include all necessary related things, like ammunition. But the right itself, though justified relative to the Militia, is absolutely an individual one.
Please note: it would be lawful for Congress to require persons eligible for service with the Militia but ALL current federal gun laws do not do this and they exceed Congress’ delegated power as it touches on anyplace within the Jurisdiction of a State.
Now, about the spurious notion of incorporation: the purpose of the 14th’s “privileges or immunities” clause was to delegate a Power to Congress to legislate federal civil rights that the several States (State actors) could not disparage. It was NOT added for incorporation. This is actually easily proven.
But “incorporation” is also not necessary because in A4:S2:C1 the States were already required to continue to honor the Privileges and Immunities that, at a minimum, they were already honoring among all of them at the time the Constitution was Ratified (this leaves out arguments arising from common or natural law). The right to bear Arms was absolutely in common among all the several States that Ratified and indeed they would still be colonies if it was not for this right. This too was for Arms usable for common military service, even light canon, and so all State laws preventing ordinary Citizens from actually owning Arms suitable for current infantry are also excessive to what they should lawfully be able to do.
It is important to argue the correct part of the Law.
As for what are NOT the P&I the States are to honor or likewise what are NOT among the other rights retained per the 9th Amendment, well, that is actually fairly easy to circumstantially demonstrate: anything unlawful in any of the several States that Ratified is not a P&I … never mind something generally unlawful (as homosexual acts or adultery generally were).