So this militiaman walks into the Senate with a loaded gun...

Now follow this fairly closely.

Phillip Thompson carried a gun and two loaded magazines into the US Senate building and got busted:

Thompson apparently put the gun in his briefcase, forgot he had it, drove into D.C., and was busted after walking through an X-ray machine at the entrance to the Russell Senate Office Building.

Not for violation of any US federal security breach, mind you, but because he violated the Washington District local law prohibiting the carrying of an unlicensed pistol outside the home or place of business (see photocopy of criminal complaint here). I guess no terrorist laws are implicated by taking a gun into the Senate.

Well, to use the Seinfeld defense: Not that there's anything wrong with that. The United States Court of Appeals has ruled that the Washington District prohibition is unconstitutional (though whether in this particular fact situation may still be debatable).

According to the United States Court of Appeals,

"[T]he District’s virtual ban on handgun ownership is
not based on any militia purpose. It is justified solely as a measure to
protect public safety. As amici point out, and as D.C. judges are well
aware, the black market for handguns in the District is so strong that
handguns are readily available (probably at little premium) to
criminals. It is asserted, therefore, that the D.C. gun control laws
irrationally prevent only law abiding citizens from owning handguns.
It is unnecessary to consider that point, for we think the D.C. laws
impermissibly deny Second Amendment rights."

"Once it is determined — as we have done — that handguns are “Arms”
referred to in the Second Amendment, it is not open to the
District to ban them.... Indeed, the pistol is the
most preferred firearm in the nation to “keep” and use for
protection of one’s home and family.... And, as we have noted, the Second
Amendment’s premise is that guns would be kept by citizens for
self-protection (and hunting)."

The Appeals Court reduced the Bill of Rights to a mere "policital document", to strip the militia reference of any operative or conditional meaning:

"We therefore take it as an expression of the drafters’ view that the
people possessed a natural right to keep and bear arms, and that
the preservation of the militia was the right’s most salient
political benefit—and thus the most appropriate to express in a
political document."

In fairness to the decision, it does canvass the debate between those who consider the second amendment "miilitia" reference to mean the "right" to keep and bear arms is a collective right resulting in the power of government to more expansively regulate guns, and those who find the militia reference only incidental to a fundamental individual right which is more strictly construed against government regulation of guns. The Appeals Court just prefers the (minority) individual rights position. It notes

"The lower courts are divided between these competing
interpretations. Federal appellate courts have largely adopted
the collective right model.4 Only the Fifth Circuit has
interpreted the Second Amendment to protect an individual
right.5 State appellate courts, whose interpretations of the U.S. Constitution are no less authoritative than those of our sister
circuits, offer a more balanced picture.6 And the United States
Department of Justice has recently adopted the individual right
model. See Op. Off. of Legal Counsel, “Whether the Second
Amendment Secures an Individual Right” (2004) available at
http://www.usdoj.gov/olc/secondamendment2.pdf; see also
Memorandum from John Ashcroft, Attorney General, to All
United States’ Attorneys (Nov. 9, 2001), reprinted in Br. for the
United States in Opposition at 26, Emerson, 536 U.S. 907 (No.
01-8780)."

"We have noted that there is no unequivocal precedent that
dictates the outcome of this case. This Court has never decided
whether the Second Amendment protects an individual or
collective right to keep and bear arms. On one occasion we
anticipated an argument about the scope of the Second
Amendment, but because the issue had not been properly raised
by appellants, we assumed the applicability of the collective
right interpretation then urged by the federal government.
Fraternal Order of Police v. United States (F.O.P. II), 173 F.3d
898, 906 (D.C. Cir. 1999). The Supreme Court has not decided
this issue either. See id. As we have said, the leading Second
Amendment case in the Supreme Court is United States v.
Miller.
...
"Few decisions of Second Amendment relevance arose in the
early decades of the twentieth century. Then came Miller, the
Supreme Court’s most thorough analysis of the Second
Amendment to date, and a decision that both sides of the current
gun control debate have claimed as their own. We agree with
the Emerson court (and the dissenting judges in the Ninth
Circuit) that Miller does not lend support to the collective right
model.... Although Miller did not explicitly
accept the individual right position, the decision implicitly
assumes that interpretation.

"On the question whether the Second Amendment protects
an individual or collective right, the Court’s opinion in Miller is
most notable for what it omits. The government’s first argument
in its Miller brief was that “the right secured by [the Second
Amendment] to the people to keep and bear arms is not one
which may be utilized for private purposes but only one which
exists where the arms are borne in the militia or some other
military organization provided for by law and intended for the
protection of the state.” Appellant’s Br. at 15, 307 U.S. 704
(No. 696). This is a version of the collective right model. Like
the Fifth Circuit, we think it is significant that the Court did not
decide the case on this, the government’s primary argument.
Emerson, 270 F.3d at 222. Rather, the Court followed the logic
of the government’s secondary position, which was that a shortbarreled
shotgun was not within the scope of the term “Arms”
in the Second Amendment....

"The government’s weapons-based argument
provided the Miller Court with an alternative means to uphold
the National Firearms Act even if the Court disagreed with the
government’s collective right argument. The Miller Court’s
holding is based on the government’s alternative position:

"In the absence of any evidence tending to show
that possession or use of a “shotgun having a barrel of
less than eighteen inches in length” at this time has
some reasonable relationship to the preservation or
efficiency of a well regulated militia, we cannot say
that the Second Amendment guarantees the right to
keep and bear such an instrument. Certainly it is not
within judicial notice that this weapon is any part of
the ordinary military equipment or that its use could
contribute to the common defense. Aymette v. State, 2
Humphreys (Tenn.) 154, 158."
Miller, 307 U.S. at 178 (emphasis added).

"The quotation makes
apparent that the Court was focused only on what arms are
protected by the Second Amendment, see Emerson, 270 F.3d at
224, and not the collective or individual nature of the right.
...
"Thus the Miller
Court limited the term “Arms”—interpreting it in a manner
consistent with the Amendment’s underlying civic purpose.
Only “Arms” whose “use or possession . . . has some reasonable
relationship to the preservation or efficiency of a well regulated
militia,” id. at 177, would qualify for protection."

So, the result of the analysis is that arms suitable for a militia-like defense, which surely would include military weapons such as AR-15s (and somehow tiny peashooting concealable handguns fall into this category), get second amendment privilege, but ones that are by some stretch of characterisation not of that ilk, such as a sawed-off shotgun, would not find second amendment protection. This seems to Guambat to be an unsatisfactory line to draw.

Guambat reckons the second amendment will end up being a right to bear any arm except a sawed-off shotgun.

Oops, since saw-off shotguns are used as riot guns for the police and in combat for the military, maybe even sawed-off shotguns are appropriate for the militia. So much for the Supreme Court Miller case bright lines.

See Dana Milbank's impressions of the Thompson case here.