A man conducting what he says was a "social experiment" has recently been arrested on charges of making a terrorist threat. He told detectives that he wanted to know how seriously our society, including the government, takes the 2nd Amendment, and got his answer when he walked into a Walmart store openly carrying an assault rifle and was immediately arrested and told that he could face up to four years in prison for threatening to commit domestic terrorism. This is despite the fact that the state in which he lives (Missouri) is an open carry state, and despite the fact that Walmart has no official policy on whether customers may openly carry in their stores (and which in fact sells guns), and despite the fact that he never pointed his gun at anyone or made any remarks implying a desire to carry out violence. He broke no laws, apart from the "terroristic threat" he made by exercising his Constitutional right to bear arms.
On the same day this incident occurred, a self-described anti-gun rights activist was questioned by detectives for performing a similar stunt, in which he asked a Walmart salesperson for something "that would kill 200 people." Apparently, police were more sympathetic to the motive behind his stunt, because he was let go without charges.
It should be clear that any laws the first man might have broken by exercising his 2nd Amendment rights can have no legal legitimacy in light of the 2nd Amendment's unambiguous proscription against governmental actions that infringe a citizen's "right to keep and bear arms." Note that the 2nd Amendment does NOT say that a citizen's right to bear arms are respected so long as he is permitted to own some types of guns and can bear them under specific circumstances. The framers could have phrased the amendment in any manner they wished, and they went with "shall not be infringed" over some weaker phrase such as "shall be respected in some manner." The meaning of this is clear, despite the criminal attempts of our courts to obscure it. The US government is simply not allowed to jail its citizens for walking around with guns of a certain type, or to insist on pushing us down any other slippery slope in this regard.
If it's impossible to exercise one's Constitutional rights without scaring others because such displays are so rare these days, then perhaps society is to blame rather than the man in question. If you're more scared of men like him than you are of a government which feels that it has the right to ignore its own laws, even one so sacred as to earn the second spot on the Bill of Rights, then you may want to brush up on your math skills.
You see, mass shooters do not (and really, cannot) pose a serious security risk to this or any other nation. They will never be responsible for more than a tiny fraction of deaths in any society, and will never affect the destiny of civilization except to the extent that we foolishly allow them to. If you add up all the deaths from all the mass shootings carried out in the US during the last 100 years, it adds up to around one pretty bad plane crash. Not even the worst plane crash, but just a pretty bad one (under 500 deaths). Governments, on the other hand, have been known to kill their citizens by the hundreds of millions. Thus, it would be closer to the truth to say that no American has ever died from a mass shooting than to say that so many have died that it's time to let the government oppress us. Indeed, 500 is a lot closer to 0 than it is to 100,000,000.