The brief in the first place is an excellent review of origin meaning and application of the 2nd amendment which everyone should read. Second, all it requests is that the Supreme Court clarify the standard of review relevant to this case and remand back to the lower courts for rehearing under the rules established by the Supreme Court. This is neither unusual nor unreasonable.
The brief contains the lie that Machine Guns are unusually dangerous to the public safety. From 1934-1986, one could go out and buy a brand new machine gun at a reasonable price, as long as you passed a tough background check and paid a $200 tax. During that time (which Parker would return to) there was only one (1) crime committed with a lawfully possessed machine gun, and that was by a police officer.
I've posted several times pointing out how weak the US brief is and that it isn't much of a threat to us.
For correctness sake, however, it should be pointed out that the US brief specifically asks that the Supreme Court ADOPT a different standard, not just clarify, and that standard would permit the outlawing of an entire category of firearms, specifically machineguns. Unfortunately, the result of permitting the outlawing of machineguns would be a level of scrutiny that would permit outlawing of handguns.
As an example of the weakness of the government's brief, I point out that the brief contains the word "infringed" only twice; once when quoting the text of the Second Amendment and once when quoting an older court decision which found that banning concealed carry is not an infringement.
Adopting the standard requested by the government would be no different than treating the Second Amendment as if it didn't exist. If the only protection afforded is that of the pre-existing common-law right to defend self and community, then there would have been no need for an amendment to be ratified. "Shall not be infringed" cannot possibly be interpreted as "shall be reasonably regulated as Congress sees fit". It was intended, at the very least, to be equivalent to "Congress shall make no law ...".