“A lot of people didn’t even know there was a permit system,” said Kevin L. Jamison, a lawyer in Missouri who concentrates on weapons cases.

Professor Webster said there was no way to tell for sure that the old system worked, because there was no central repository for permits, but that the rise in crime guns originally purchased in Missouri suggests that the previous law was effective.

The changes have complicated the daily work of the police. Col. D. Samuel Dotson III, the police chief in St. Louis, said a recent change that made it legal to carry a firearm in a vehicle without a permit had made it harder for officers to connect an illegal gun to a person during a traffic stop. Professor Rosenfeld, of the University of Missouri, analyzed hundreds of arrests involving guns from 2011 and found that 40 percent of the cases were never prosecuted, because the change made it harder to prove the charges.

Guns also confer status. And because the repercussions for carrying them without the proper authorization are so minor, there is little risk in showing them off.

“The big thing used to be having a fancy car and driving it around,” Chief Dotson said. “Now, it’s having a pistol with extended magazine and posting pictures of it on Facebook.”

Jennifer M. Joyce, the top prosecutor in St. Louis, keeps a framed picture of a teenager from one of her recent cases posing with a gun.

“It has reached the point in St. Louis where this guy can get a gun really easily,” she said. “He believes that gun is going to allow him to avenge the death of his buddy. But what we all know is that gun will either be the death of him or cause him to end up in prison.”