There are a wide variety of myths and urban legends around the US about weapon laws. Many are very common among the general population, and some are even held by police officers! This page will clear up some the most common ones.
Bigger than the "Palm of the hand"
Arguably one of the most well-known and prolific myths is that a knife is illegal to carry if the blade is larger than the palm of one's hand. Variations include that it is your own hand, the hand of the police officer examining, or the judge. And therein lies the ludicrousness of this misconception: People all have different sized hands! There is no such law anywhere in any part of the US, and likely the world, yet this myth is strangely persistent and can even be found among police officers and security guards.
This item is illegal in the United States
Almost always an incomplete belief or statement. The 50 states of the US each has different weapon laws to the point where one could almost call each state it's own sovereign nation. Some states such as New York and Massachusetts have extremely restrictive weapon laws, while states like Vermont and Alaska allow citizens to carry handguns without any permit being required. Furthermore, some states only have laws on carrying an item in public, but no law on owning at home.
Switchblades and Balisongs (butterfly knives) are illegal everywhere without exception
Not true. Most laws about non-firearm weapons are by state, with very few existing at the Federal level. Currently, the only country-wide law about switchblades is US Code Title 15, Chapter 29, and this law only controls the importation of these knives into the US, and the sale over state lines. It has no effects on buying, owning or carrying switchblades. Such is left to the states. The majority of states allow legal ownership of switchblades, but a few do not. Many have prohibitions on the sale of them, and most have laws against carrying them concealed. But some states such as Vermont and Utah have no restrictions on them.
Balisongs (butterfly knives) and switchblades are illegal because they can be opened faster
False. A non-folding knife is always "open" and therefore is technically "faster" than any butterfly knife or switchblade. The reasons for the laws about these knives are almost entirely political and emotional in nature. Also, as a general rule, most one-handed folding knives, such as those that use thumb-studs, are still faster to open than a balisong. Simple mechanics: The conventional folder only move 180 degrees on one axis and requires one arcing motion. The balisong turns 360 degrees on one axis and 180 degrees on another, and requires a series of movements with both the arm and multiple fingers to complete.
You can buy a switchblade if you work in law enforcement or you are in the US military.
If buying from a seller out of state, wrong. The federal law on switchblades states very clearly in its exceptions that the ban on intrastate sale does not apply "pursuant to contract with the Armed Forces." First, note there is no mention of law enforcement anywhere in the law. Second, the key word is "contract." Just being a member of the military does not give one the ability to buy whatever you want when off-duty. Try that with rocket launcher! The law only exempts purchases made when an official contract has been made between the US government and the knife manufacture/seller.
You can carry any weapon you want if you have a Concealed Weapon Permit
Not always true, and it depends on the state. Most states only apply these permits to handguns, and do not call their permits "Concealed Weapon Permits" as "weapon" is too vague and they are invariably issued for purposes of carrying a handgun. Most call the permit a "Concealed Handgun Permit," "Concealed Pistol License" or variations on those. And those that do call it a "Weapon Permit" do not necessarily allow other weapons besides a handgun in the law itself, so be wary. Why would a permit legally allow you to carry a gun and not something far less deadly? Who said laws make sense!