WATCH: Judge Nap's Prediction on New 2nd Amendment Case in Supreme Court
The central issue is whether someone should be able to buy a firearm for someone else if both are legally allowed to own a gun.
The case stems from an attempt in 2009 by a former police officer to buy a discounted gun for his uncle.
The federal government likened the purchase to a "straw buy," in which a person who can legally purchase a gun does so for a criminal.
Napolitano explained that in this case, the gun could have only been used in the home for self-defense, not for concealed-carry.
"The home, in American history and in American law and under the Constitution, is a sacred place, where the law doesn't always intrude ... It's a gun to be used in the home. Does the federal government really want to intrude that deeply in our lives?" he asked.
The judge compared it to a situation where a person is prescribed a drug for the flu, then their child gets sick so they also take the medicine.
His prediction? The justices will toss the conviction in a 5-4 ruling.