Texas defendant challenging federal gun law at the Supreme Court says he doesn’t want firearms anymore

That isn't a constitutional argument, that is a stare decisis argument.

(1) The Second Amendment protects an individual right to possess a firearm unconnected with service in a militia, and to use that arm for traditionally lawful purposes, such as self-defense within the home. Pp. 2–53.

(a) The Amendment's prefatory clause announces a purpose, but does not limit or expand the scope of the second part, the operative clause. The operative clause's text and history demonstrate that it connotes an individual right to keep and bear arms. Pp. 2–22.

(b) The prefatory clause comports with the Court's interpretation of the operative clause. The "militia" comprised all males physically capable of acting in concert for the common defense. The Antifederalists feared that the Federal Government would disarm the people in order to disable this citizens' militia, enabling a politicized standing army or a select militia to rule. The response was to deny Congress power to abridge the ancient right of individuals to keep and bear arms, so that the ideal of a citizens' militia would be preserved. Pp. 22–28.

Here is what is said above that, where it is clearly a constitutional argument, because it references the constitution as evidence- not other courts.

I'll buy that when one these "conservative legal scholars" aka moron federalists can produce a single primary source document that uses the phrase bear arms outside of a strictly military context involving uniformed, regimented troops, and instead refers expressly to an individual right of self defense.

Otherwise, it's not ambiguous.

Well he's right about the able bodied bit at least. Under the legal definition of militia in the US every able bodied male age 17-45 is actively part of the militia. One could make the argument that women (outside the NG), disabled people, and old people shouldn't have the right to guns with this I suppose but that feels sexist, ableist, and ageist (is that still a thing?)

"(a)The militia of the United States consists of all able-bodied males at least 17 years of age and, except as provided in section 313 of title 32, under 45 years of age who are, or who have made a declaration of intention to become, citizens of the United States and of female citizens of the United States who are members of the National Guard." https://www.law.cornell.edu/uscode/text/10/246#:~:text=(a)The,the%20National%20Guard.

The wording while ambiguous to some seems fairly clear to me tbh too, "the right of the people to keep and bear arms" is the only part that on its own makes a complete sentance, the prescriptive clause simply explaining that the reason "the people" have the right to keep and bear arms is because "the militia" as defined above is important. "Well regulated" is the only bit one could quibble over, however as "the militia" is defined, it seems clear to be intending "well maintained" as per historical definitions of "well regulated," though historically it was used both as that and how we'd use it today.

https://www.oed.com/search/advanced/Quotations?textTermText0=Well+regulated&textTermOpt0=QuotText

Furthermore, from other historical papers that might offer context, we have quotes like these:

“No free man shall ever be debarred the use of arms.” – Thomas Jefferson, Virginia Constitution, Draft 1, 1776

“What country can preserve its liberties if their rulers are not warned from time to time that their people preserve the spirit of resistance. Let them take arms.” – Thomas Jefferson, letter to James Madison, December 20, 1787

“The laws that forbid the carrying of arms are laws of such a nature. They disarm only those who are neither inclined nor determined to commit crimes…. Such laws make things worse for the assaulted and better for the assailants; they serve rather to encourage than to prevent homicides, for an unarmed man may be attacked with greater confidence than an armed man.” – Thomas Jefferson, Commonplace Book (quoting 18th century criminologist Cesare Beccaria), 1774-1776

“A strong body makes the mind strong. As to the species of exercises, I advise the gun. While this gives moderate exercise to the body, it gives boldness, enterprise and independence to the mind. Games played with the ball, and others of that nature, are too violent for the body and stamp no character on the mind. Let your gun therefore be your constant companion of your walks.” – Thomas Jefferson, letter to Peter Carr, August 19, 1785

“The Constitution of most of our states (and of the United States) assert that all power is inherent in the people; that they may exercise it by themselves; that it is their right and duty to be at all times armed.” – Thomas Jefferson, letter to John Cartwright, 5 June 1824

“On every occasion [of Constitutional interpretation] let us carry ourselves back to the time when the Constitution was adopted, recollect the spirit manifested in the debates, and instead of trying [to force] what meaning may be squeezed out of the text, or invented against it, [instead let us] conform to the probable one in which it was passed.” – Thomas Jefferson, letter to William Johnson, 12 June 1823

“They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety.” – Benjamin Franklin, Historical Review of Pennsylvania, 1759

“To disarm the people…[i]s the most effectual way to enslave them.” – George Mason, referencing advice given to the British Parliament by Pennsylvania governor Sir William Keith, The Debates in the Several State Conventions on the Adooption of the Federal Constitution, June 14, 1788

“I ask who are the militia? They consist now of the whole people, except a few public officers.” – George Mason, Address to the Virginia Ratifying Convention, June 4, 1788

“Before a standing army can rule, the people must be disarmed, as they are in almost every country in Europe. The supreme power in America cannot enforce unjust laws by the sword; because the whole body of the people are armed, and constitute a force superior to any band of regular troops.” – Noah Webster, An Examination of the Leading Principles of the Federal Constitution, October 10, 1787

“A militia when properly formed are in fact the people themselves…and include, according to the past and general usuage of the states, all men capable of bearing arms… “To preserve liberty, it is essential that the whole body of the people always possess arms, and be taught alike, especially when young, how to use them.” – Richard Henry Lee, Federal Farmer No. 18, January 25, 1788

“Guard with jealous attention the public liberty. Suspect everyone who approaches that jewel. Unfortunately, nothing will preserve it but downright force. Whenever you give up that force, you are ruined…. The great object is that every man be armed. Everyone who is able might have a gun.” – Patrick Henry, Speech to the Virginia Ratifying Convention, June 5, 1778

“This may be considered as the true palladium of liberty…. The right of self defense is the first law of nature: in most governments it has been the study of rulers to confine this right within the narrowest limits possible. Wherever standing armies are kept up, and the right of the people to keep and bear arms is, under any color or pretext whatsoever, prohibited, liberty, if not already annihilated, is on the brink of destruction.” – St. George Tucker, Blackstone’s Commentaries on the Laws of England, 1803

I think it's pretty clear what they intended. Not to say you can't disagree with their intentions themselves, but it does seem pretty clear what they did intend.

It is far from ambiguous. The first half tells you why the right exists and only part of why. The second half is the right itself, which is the right of the people to keep and bear arms.

Security of a free state.

By justification I mean the reason for the right. The right being the right to bear arms.