The most recent precedent from the Court on this issue comes in the form of District of Columbia v. Heller and McDonald v. Chicago which were decided in 2008 and 2010 respectively. The holding in each case was substantively similar as McDonald primarily served to incorporate the 2nd Amendment via the 14th Amendment (since Heller applied to the District of Columbia only which is governed by the federal government directly) to the states and did not really set new gun ownership precedent.
Heller dealt with a strict District of Columbia law that banned ownership of any handgun in the district and also required that any gun (even rifles and shotguns) should be kept unloaded and under the control of a trigger lock at all times when not in use. The Court's ruling went as follows:
- The clause "A well regulated Militia, being necessary to the security of a free State" is merely colorful language and has no legal meaning. It does not expand or limit the scope of the following clause, which should be taken on its own.
- The right to bear arms as stated in the Second Amendment is not unlimited. Congress has the right to limit the manner and intent that they are used. Indeed, the Court upholds prohibitions on felons and the mentally ill owning guns explicitly in the decision.
- Handguns are "arms" for the purposes of the 2nd Amendment. Since there are lawful uses of arms (as described in the 2nd Amendment - i.e. the natural right of defense) no class of arms can be arbitrarily banned in all cases and for all uses.
- The requirement that all guns be unloaded, disassembled and/or under the control of a trigger lock while not in use has a direct, negative, effect on a legitimate lawful use of a protected "arm". As such, Congress does not have even a rational basis to implement this particular type of limitation on gun ownership.
In McDonald the Court extrapolated these same requirements to the states.
Importantly, the Court did not overturn existing precedent that some in the camp favoring a limit to gun ownership rights point to as an indication that the militia clause is important, but rather found a way to uphold that precedent by clarifying its interpretation. In United States v. Miller the Court upheld limits in the type of legal firearms found in the National Firearms Act of 1934 (specifically fully automatic rifles and short-barreled shotguns) because they would not be needed by a well regulated militia to efficiently carry out their duties.
However, the Court explained in Heller that the Miller precedent from 1939 is the same precedent at work in Heller in 2008. This is because the Court in Miller was not claiming that a well regulated militia is required for gun ownership, but rather that it can be used as a guide as to the appropriate manner and intent of individual gun ownership. That the Court squared their ruling in Heller with the existing precedent in Miller, that could be seen to underline the importance of the militia clause, the Court actually broadened the ruling in Heller even further than it otherwise would have been if it just overturned the ruling in Miller.
Applying this precedent going forward is squarely a job for the Supreme Court because even though some limitations on gun ownership might seem directly in line with the Court's thinking in Heller those specific provisions will still need to undergo at least a rational basis test on its own merits. For example, this reasoning might sound like it would support the limiting of high capacity magazines as has been recently discussed in the media because it would not be required for a militia to efficiently perform its duties (Miller) and is only a limit on the manner and intent of the use of otherwise legal arms (Heller). However, Congress would have to demonstrate at least a rational basis (perhaps even more in the view of the Court at that time) to justify the law and that interpretation will need to be made by the Court at that time on its individual merits. Until that day, this existing case law is the best guide we have for projecting the viability of future gun laws.