Want to bet? This is from the Heller opinion, which McDonald restated:
2. Like most rights, the Second Amendment right is not unlimited. It is not a right to keep and carry any weapon whatsoever in any manner whatsoever and for whatever purpose: For example, concealed weapons prohibitions have been upheld under the Amendment or state analogues. The Courts opinion should not be taken to cast doubt on longstanding prohibitions on the possession of firearms by felons and the mentally ill, or laws forbidding the carrying of firearms in sensitive places such as schools and government buildings, or laws imposing conditions and qualifications on the commercial sale of arms. Millers holding that the sorts of weapons protected are those in common use at the time finds support in the historical tradition of prohibiting the carrying of dangerous and unusual weapons. Pp. 5456.
Obviously, you do not know how to read a decision from SCOTUS. See if this makes any sense to you:
In those cases, rational basis is not just the standard of scrutiny, but the very substance of the constitutional guarantee. Obviously, the same test could not be used to evaluate the extent to which a legislature may regulate a specific, enumerated right, be it the freedom of speech, the guarantee against double jeopardy, the right to counsel, or the right to keep and bear arms.
Id at 56
If it does not make any sense to you, there are 3 levels of scrutiny which SCOTUS applies when evaluating the constitutionality of a law. Rational basis is the standard employed when no fundamental right is involved or no suspect or quasii suspect class is singled out. Heightened scrutiny is employed when a quasi suspect class, such as age or gender is implicated or a non core area of a fundamental right is restricted. An example would be that free speech may be subject to time, place and manner restrictions which are content neutral, provide an alternate effective venue when those restricitons serve an important state interest and are properly confined so as to serve those important state interest, such as not allowing a march on the interstate freeway during rush hour. Strict scrutiny requires a compelling state interest which is tightly constrained in scope so as to serve only the compelling state interest.
SCOTUS specifically reject rational basis scrutiny in favor of either heightened scrutiny or strict scrutiny. So your assertion is totally bogus.
Scalia says in the footnote rational basis is not the test, yet his examples of what regulation is permissible meet the rational test, and he does not apply anything more stringent to them. Basicaly it's left to another day.
Like most rights, the right secured by the Second
Amendment is not unlimited. From Blackstone through
the 19th-century cases, commentators and courts routinely
explained that the right was not a right to keep and
carry any weapon whatsoever in any manner whatsoever
and for whatever purpose. See, e.g., Sheldon, in 5 Blume
346; Rawle 123; Pomeroy 152153; Abbott 333. For example,
the majority of the 19th-century courts to consider the
question held that prohibitions on carrying concealed
weapons were lawful under the Second Amendment or
state analogues. See, e.g., State v. Chandler, 5 La. Ann.,
at 489490; Nunn v. State, 1 Ga., at 251; see generally 2
Kent *340, n. 2; The American Students Blackstone 84, n.
11 (G. Chase ed. 1884). Although we do not undertake an
exhaustive historical analysis today of the full scope of the
Second Amendment, nothing in our opinion should be
taken to cast doubt on longstanding prohibitions on the
possession of firearms by felons and the mentally ill, or
laws forbidding the carrying of firearms in sensitive places
such as schools and government buildings, or laws imposing
conditions and qualifications on the commercial sale of
Cite as: 554 U. S. ____ (2008) 55
Opinion of the Court
arms.26
We also recognize another important limitation on the
right to keep and carry arms. Miller said, as we have
explained, that the sorts of weapons protected were those
in common use at the time. 307 U. S., at 179. We think
that limitation is fairly supported by the historical tradition
of prohibiting the carrying of dangerous and unusual
weapons.
Is a background check the least intrusive means to keep guns from the mentally ill or felons? I have no idea. Obviously, is less than 100% effective, yet Scalia says sales can be regulated. Magazine capacity would not have a lot of impact on most murders in Chicago, yet unusual weapons can be restricted.
Perhaps the ambiguity is Scalia being ambiguous to these tests, and he got his biggest objective of an individual right with five votes, and decided to quit while he was ahead.