
Amongst other definitions, Heller highlighted that the founding era understood the term [arms] to mean “any thing that a man wears for his defence, or takes into his hands, or useth in wrath to cast at or strike another.” Switchblade knives have surely been used for “defence” and to “strike another.”
Knives have been used for defense throughout human history, including the American founding. During the colonial period, “[e]veryone had a knife,” and “as long as a man was required to defend his life,” a “knife was a constant necessity.” In the 1700s and during the American Revolution, knives were used for scavenging, hunting, and defense, and “[b]elt and pocket knives” accompanied frontiersmen “in the developing eastern settlements and on the western frontiers.” Well into the 1800s, “men of all walks of life” were “accustomed to wear a knife as a necessary part of their garb.”
Switchblade knives are no exception. “Springblade knives” (precursors to the modern switchblade) “have been made for hundreds of years because there has always been a need for a pocketknife that can be opened easily when the hands are wet, cold, or covered with gloves.” In the 1700s, such knives were a part of certain types of flint-lock pistols: “[i]f the pistol mis-fired (which frequently happened) the person would touch the release button and the blade would open up, and be available for use.” After the Civil War, patents for switchblade knives proliferated, and the large-scale production of “switchblades in the United States began in the 1880s, shortly” thereafter. Then, “[o]ver a 50 year period from the mid 1890s to the mid 1940s there had been approximately 20 different companies who had manufactured switchblade knives in this country.” In short, some forms of switchblades have been commonly used for defense for hundreds of years. They are “arms.”
Accordingly, the government’s not-in-common-use argument fails. In addition to the wealth of history indicating the founding-era uses of switchblades for defense, the record indicates that there are “millions” of automatically opening knives in the United States now. The record further reflects that switchblade knives are lawfully used for “every day utility use, hunting, fishing, recreation, and camping and hiking.” Despite the FSA, there are still lawful sales of switchblade knives in the United States. Consequently, switchblade knives are “arms” “in common use” both then and now.
— Fifth Circuit Ruling in Knife Rights v. Blanche


A slingshot is an Arm. And a deadly one in the 21st-century. The nunchuck is an Arm. And several hundred years old. The 2A has never ever been just about guns.
Please read.
By Joyce Lee Malcolm
“To Keep and Bear Arms: The Origins of an Anglo-American Right” 1994
The 1958 Federal Switchblade Act was passed by a bunch of federal legislators from New York who wet their pants after watching the Tony Award winning 1957 musical West Side Story. There never was any criminal use of switchblades.