Folks looking to the Courts to save us or to extend 2A rights are looking the wrong place. Part of the issue is that since Carolene Products, the focus of judicial review has primarily been to strike down state and local laws that affect matters of identity--race, ethnic origin, etc., not economic or criminal laws. In Carolene Products, the footnote 4 provides that "review of statutes directed at particular religious . . . or national . . . or racial minorities . . . : whether prejudice against discrete and insular minorities may be a special condition, which tends seriously to curtail the operation of those political processes ordinarily to be relied upon to protect minorities, and which may call for a correspondingly more searching judicial inquiry." The backlash against substantive judicial process such as the Court used in Lochner (which struck down minimum wage and worker protection laws in NY), has more or less meant that the Court uses the presumptively constitutional framework for most constitutional challenges of laws that doe not deal with questions of race and identity. The Court also uses judicial review far more on state and local laws than it ever did on federal laws.
Thus, the question on most firearms regulation because of precedent like Miller, is that they are presumptively constitutional, (the 1939 Miller case involved a reversal of the district court's decision that the NFA was unconstitutional for example.) Justices also dislike disturbing old precedents and laws, particularly when it clips the powers of the federal government.
Using the prior decisions of Miller and Heller as precedents, the district court held that A) the NFA was constitutional exercise of Congressional taxing power and B) that a silencer is not a bearable arm under the 2A as it is not in common use. Heller's use of Miller's language of common use is to blame. Currently there is no conflict among appellate courts nor district courts on the legality of the NFA and thus it is regarded as settled law. Generally, as seen in NFIB v. Sebelius, the congressional power to tax is almost plenary (unreviewable by the court). As the NFA was passed using Congressional Taxing powers rather than commerce powers, the Kansas law was not considered as a challenge to congressional taxing power but rather commerce clause regulation.
If you read the US govt's filing to deny cert, it is for two reasons. 1) the idea that someone can rely on a state law that is contrary to federal law to avoid federal prosecution, and 2) there is no current disagreement among the courts. Generally speaking it is the duty of the solicitor general to defend the constitutionality of federal statutes as otherwise, abandonment of that principle would mean that every law could be potentially overturned by courts when an administration did not like some old laws. This was an old problem when the law was whatever the king said and thus when a king died, those laws were in question anew. Thus, the idea of a rule of law is that unless exigent or extraordinary circumstances exist, laws should be defended in court whether or not a current government agrees with them. Note that no justice felt strongly enough about this denial of cert to file a dissent. This means that the constitutionality of the NFA is current regarded as settled by most court members if not all. Thus, you do not want such a court to potentially extend federal powers to regulate firearms under the NFA.
The petitioner Kettler, had some interesting arguments in his cert petition on the use of the taxing power in the NFA but as this would require a significant new ruling by the court overriding Supreme Court precedent on the extent of congressional taxing power. Petitioner Shane Cox asked the court to allow the Kansas state law to be raised as a defense to his convictions of making an unregistered short barrel rifle, several suppressors, etc. He also tried to raise the 2A as a defense claiming by asking the Court to basically overrule Miller and its use in Heller. However, Cox did not persuade the appellate court to overrule Supreme Court precedent in Miller and Heller regarding in common use provisions.
The petitioners in this case (the defendants in the criminal case) were simply stupid in doing acts that they knew or reasonably should have known were illegal under federal law (making a suppressor and selling it for one and buying this suppressor by the other.) We also see claims that because other people in Kansas allegedly were equally ignorant of the law, that it should be able to be a defense.
Generally speaking, ignorance of the law is not a viable defense. They claimed as a defense that a Kansas law passed made their conduct legal and they claimed they were misled by state authorities that passed the Kansas law and raised a "defense of entrapment by estoppel". However, the long settled doctrine is that only if the defendants relied on a person enforcing the law (in this case the NFA) could this defense be employed. Thus, if they had relied on the ATF to issue a decision that the Kansas law superceded the federal NFA law, they could employ this defense. The district court allowed the defendants to raise the state law as a mitigating issue in the case but not as to whether or not they should be held guilty.
Folks, if you want things like suppressors, automatic or select fire firearms, or short barrelled rifles to be legal without the folderol of the NFA or felons to regain firearm rights, regardless of whether you personally believe that the 2A protects these things, currently most of the public does not. Currently, the federal courts, legislature, or even the executive demonstrate little agreement with that position. Thus, expecting the courts to rule for such things is like spitting into the wind.
Instead, one--take advantage that most suppressors go for about the same amount as a new quality firearm. Consider instead of buying the new hot "firearm" in the gun mags and forums to buy a suppressor legally under the NFA. Ditto for a SBR. Undermine the law by establishing new facts on the ground. Activists in states have gradually reduced the number of states that ban suppressors to about eight states the hard way by persuading legislators and governors in these states that the benefits outweigh potential illegal uses.
In a similar way, work to raise the issue with Congress critters as ultimately they hold the fate of the NFA and whether or not it can be changed. To do this, figure out what arguments work for your particular Congress critter or primary candidate. On suppressors, most of them will be quite ignorant on the subject and most of their knowledge will come from movies and tv. Simply demonstrating that these things are not noiseless on high powered rifles might sway some along with the Hearing Protection Act testimony and evidence. Be ready for the terrorism and criminal use question and why it is needed in concrete terms.