Last Updated Jul 11, 2016 5:24 PM EDT
Due to this status, which the SKS shares with many other models of Berettas, Colts, Remingtons, Rugers and other firearms that are at least 50 years old, gun dealers said that in some states and jurisdictions the Soviet-era rifle can be purchased online and delivered to your door without securing a permit.
In the hands of Johnson, a trained shooter and army reservist who had private firearms training and even kept a combat manual on how to "shoot and scoot," the semi-automatic SKS that the FBI initially said he used proved as deadly and accurate that night as it had in Vietnam, when it killed hundreds of U.S. soldiers. (Update: There are conflicting reports about about what kind of rifle Johnson used in the shooting. Citing an unnamed law enforcement official, The Wall Street Journal said Monday that he used a different weapon, an Izhmash-Saiga model, which is based on the AK-74 military rifle.)
The SKS or "self-loading carbine Simonov," named for its Russian inventor, Sergei Simonov, was invented near the end of World War II when the Soviet army needed a gun to match the German army's Sturmgewehr, which had wreaked havoc on Allied forces by combining the rapid fire of a submachine gun with the power of a rifle.
These armies recognized the necessity for a gun that could shoot multiple times without using a bolt to reload each time, yet not require a cartridge so powerful that the recoil would knock the shooter's aim off when a number of shots were fired in a row.
The shortened 7.62 by 39 millimeter cartridge (about .30 caliber U.S.) fit the bill. Millions of these SKS rifles were produced for Eastern bloc armies, including Albania, Rumania, Russia, Yugoslavia and, later, China. But the SKS became obsolete when its better-known cousin, the AK-47 or "Kalashnikov," named after its inventor, Mikhail Kalashnikov, came into being in 1947.
The Kalashnikov fired the same cartridge but had several advantages. It was easier and cheaper to make, virtually indestructible, and fired from a 30-round removable magazine, as opposed to the SKS with its fixed 10-round magazine, making it faster to reload. Most important, it had selective fire, meaning it could fire single shots or be fully automatic. In other words, it could spray 30 rounds in a few seconds and do it over and over.
But the SKS still had its fans. It was and is a sturdy and accurate gun. Both of these guns have iron sights, but the SKS is usually considered the more accurate. "At 100 yards, holding the sight on the bottom of a military target, I can hit the black with every round," said a Bordentown, New Jersey gun dealer. In contrast, the AK-47 has loose-fitting parts and is often described as "tinny."
The SKS can also be retrofitted with aftermarket parts easily found on the Internet that allow it to fire 30 or more rounds. Buyers can also buy a 75-round magazine, although this modification is illegal in some states. Scopes are available, but normally attach to the side rather than the top of the rifle due to the way rounds are ejected, which may make the gun less accurate.
Other accessories, such as railings and flashlights, aren't easy to install but the SKS can easily be modernized with plastic parts and a pistol grip.
While the U.S. military rifle, the M-16, and its civilian counterpart, the AR-15, have won wide acceptance, they fire a .223 caliber round that's only a little bigger -- but a lot more powerful -- than a .22 found in many American homes. The heavier .30 caliber slug fired by the SKS will chew through a normal ballistic vest designed to stop pistol rounds and do far more damage than a .223, said some gun experts.
Although still being produced, the AK-47 gave way to the smaller caliber AK-74, a gun brandished by Osama Bin Laden in some photos of the organizer of the September 11 attacks. Both weapons are known as the "grim reaper" because of their death toll in revolutions and guerrilla wars around the world.
When the SKS was replaced, many of the rifles were put into boxes packed with grease to keep them lubricated and rust-free, and only used in Cold War parades, where its wood fixtures and long bayonets made it a favorite ornament for marching troops.
But in the 1990s Americans rediscovered the SKS as a sporting rifle and for target shooting. Despite resistance from the U.S. State Department, SKS models from many different countries were imported and are now available for purchase on websites with prices ranging from $200 to $800 for a new or barely used model.
It has not yet been revealed how Johnson obtained the reported SKS or any other weapon he could have used in the attack. Federal officials said he had been stockpiling weapons and explosives for at least two years.
By gun standards, the SKS is relatively cheap. You can even buy a box of 10 with cleaning kits for about $4,000. And used or yard-sale models are even cheaper. According to gun dealers, the best of the bunch come from from Eastern Europe.
Because of their technically obsolete status, you can purchase an SKS if you have a $30, three-year Curio and Relic (C&R) license from the ATF. And it's only one of many "curios" available, including such classics as the 9-millimeter Browning High Power, Lugers and some Colt .45s.
One gun dealer said a client had a Thompson submachine gun delivered, the gun made famous by the St. Valentine's Day Massacre. The gun was modified to fire only semi-automatic, meaning one shot is fired with each squeeze of the trigger, so as not to violate federal law.
The big advantage of a C&R -- these aging guns are legal for interstate transport and sale. Buyers are deemed by the ATF to be a "collector."
Some states place additional restrictions on those applying for a C&R license. For example, Illinois requires applicants to have a Firearm Owners Identification (FOID) card, which entails a background check, no felony convictions and no mental illness. But for the average applicant it takes only 15 minutes to apply for the license, according to one website in the state.
Obtaining a C&R license in other states can be trickier. Firearms advocates consider New Jersey "gun unfriendly," for example, so an application for a C&R is referred to the state police, who then pass it along to the municipality. Local officials may reject it or treat it as a traditional Federal Firearms License, which requires multiple approvals and permitting.
In contrast, neighboring Pennsylvania is considered an easy state to get a C&R. Residents can get an SKS or other weapon delivered to their home, gun dealers said. Other "right to carry" states are equally liberal.
I was at a gun range once when I heard and saw an SKS fired -- accurately -- at 100 yards. An older man in a neighboring booth came over and said he always jumped when he heard the sound of the 7.62x39 round go off. He had been in Vietnam.