A shopper in the grocery section of the Winona Walmart was struggling with the back wheel of their shopping cart on a Saturday morning a few weeks back.
Like any lazy shopper, this person continued to press and push on the cart to try to force free whatever was locking up the wheel. Suddenly, there was a loud pop! and several employees and shoppers rushed over to see what the heck just happened. See also: NRA-Certified Firearm Safety Instructor Accidentally Shoots Himself
They found a spent .22-caliber shell casing next to the cart and its slug sitting a few feet away. No one was injured.
"Ultimately because of the type of bullet it is -- a .22 rimfire -- the weight of the shopping cart and pressure of it getting dragged underneath there crushed the rim of the bullet, causing it to go off," said Winona Police Deputy Chief Tom Williams.
It's a plausible explanation because rimfire rounds are ignited when the firing pin of the gun crushes the base of the cartridge's rim. The slug did not have any grooves or striations consistent with gunfire.
The incident took place on January 17, but the police weren't notified until two days later, when a woman who was there when it happened walked into the police station and asked if they were investigating.
"I'm kind of surprised no one called to say, you know, 'Hey, a bullet just went off at Walmart,'" said Williams.
Investigators reviewed the security footage, talked with three employees who first arrived on the scene, and were able to track down and interview the person who was pushing the cart.
"Somebody must've had it in their pocket, and they either reached in their pocket or had a hole in their pocket and this round ended up on the floor," said Williams. "Then, strangely enough, it ended up getting run over and going off."