Well we can't see how the shell landed.
I want to say that primers are less predictable than what we want them to be.
I get into debates all the time with people who think primer sensitivity is a constant. That is primers are all the same and they all take the same amount of energy to set them off.
Based on frequency of slamfires reports, there is a reason why semi automatic firearms with free floating pins have reports of slamfires, and those mechanisms that have captive firing pins, reports are rare to the point of non existence.
Primer goes off in a Purse!
By Eric Pfeiffer, Yahoo! News | The Sideshow – Tue, Jun 12, 2012
A Pennsylvania woman was shot in the leg while shopping at a local department store on Tuesday. But in a nearly unbelievable twist, no gun was involved. Apparently, the woman was carrying the bullet in her purse, when it mysteriously exploded.
"She did not have a gun in her purse or on her," Montoursville Deputy Police Chief Jason Bentley told the Williamsport Sun-Gazette. Bentley said the woman, whose name has not been released to the public, "was not aware" she was carrying two or three bullets inside her purse at the time of the accident.
The 56-year-old woman was taken to a local hospital and was eventually discharged. In fact, the woman initially declined medical treatment, only heading to the Williamsport Regional Medical Center after her son reportedly encouraged her to do so.
"Something must of hit the primer of one of the bullets," Bentley said. "The bullet stayed in the purse, but its casing put a hole in the purse and caused a minor leg wound."
Bullets exploding outside of a gun are a rare occurrence but are not entirely unprecedented. In March, a bullet being used as evidence in a court case exploded in a bag and shot 20 feet across a courtroom. No one was hurt in the incident. It was surmised that the bullet exploded after its tip bounced against another bullet tip in the same evidence bag, according to the Telegram & Gazette.