You must assume all of the people have a concealed gun but for example PA have more concealed licenses than Texas (over a million), so the chances for someone with a gun to be present when a shooter walks in a store are low. Most if not all the mass shootings are in a "gun free zone".
List of cases of people stopping a mass shooting:
Newnan, Georgia, August 13, 2018
This is an amazing story that was caught on video. An employee who wasn’t even scheduled to work that day, but was simply hanging around the bar outside in the parking lot, stopped an attack that the police say would have likely resulted in multiple deaths.
Colonial Heights, Tennessee, February 13, 2019
A shooting at a dentist office in Colonial Heights, Tennessee
was stopped by Larry Seagroves, a concealed handgun permit holder. After the killer, Harry Weaver, fatally shot one person, Kelly Weaver, his wife, he turned his gun on other people in the office.
Birmingham, Alabama, October 27, 2018
A masked man entered a McDonald’s restaurant and immediately started firing his gun. A concealed handgun permit holder returned firing killing the attacker.
Louisville, Kentucky, Wednesday, October 24, 2018
The national media has picked up on this story because of the killer, Gregory Bush, is racist, but they are ignoring the fact that a white civilian with a concealed handgun permit stopped the attack. Both the people murdered at the Koger — Maurice Stallard, 69, and Vickie Jones, 67 — were black. Bush had
apparently tried to do an attack at a black church before shooting blacks at the Kroger indicates he was trying to kill as many blacks as possible and that the number of blacks killed could have been much worse if it wasn’t for the concealed handgun permit holder.
Titusville, Florida, August 4, 2018
A shooting at a back to school event for children and their families had a shooting that was quickly stopped by a concealed handgun permit holder.
Tumwater, Washington, June 18, 2018
“
He is a hero,” Adams said. “He took action. I really think more people could have been shot… he saved us all.”
A wild rampage at a Walmart in Tumwater, Washington was stopped when three concealed handgun permit holders confronted and killed the attacker. The attacker had carjacked a vehicle that he drove to the Walmart. Once there, the attacker stole ammunition and then went out of the store, continuing to shoot at people.
Oklahoma City, Oklahoma, Thursday, May 24, 2018
Two men who had handguns in their cars, Juan Carlos Nazario, 35, and Bryan Whittle, 39, stopped what likely would have been a mass shooting at Louie’s Grill & Bar in Oklahoma City.
San Antonio, Texas, December 7, 2017 (ABC 13 in San Antonio, Time Magazine)
This case is an exception to all the other cases shown here in that it received national coverage on
Time magazine’s website. A man with a gun was about to start shooting a father’s children as they were walking out of a restroom when the father killed the attacker.
Rockledge, Florida, November 17, 2017 (Florida Today, WFTV.com in Orlando)
Two concealed handgun permit holders stopped an attack at Schlenker Automotive repair shop at 4:30 PM on the Friday after Thanksgiving. The attacker, Robert Bailey, was armed with
a .45-caliber handgun and “multiple magazines” and killed one person and severely wounded another in the business’ parking lot. Bailey continued shooting
inside the shop.
Clearlake Oaks, California, October 23, 2017 (FBI report on active shooter incidents for 2016-17)
From the FBI report on page 5. At the first target, the killer was forced to stop his attack because of police arriving at the scene. At the second target, a concealed handgun permit holder forced the attacker to break off his attack.
Antioch, Tennessee, September 24, 2017 (Chicago Tribune, Daily Caller) (the above picture shows a picture of Samson after he received treatment for his minor wound)
A 25-year-old shooter, Emanuel Kidega Samson, killed one person in the parking lot, entered the church, and started shooting. A 22-year-old usher, Robert Engle, tried to wrestle with the shooter, but Samson is a very large bodybuilder, and Engle realized that he was not strong enough to stop him. During the scuffle, Samson pistol wiped Engle causing him to bleed from his head. Samson also shot himself, causing very minor wounds to his chest and arm. Engle then ran to his car and got his permitted concealed handgun. The usher came back into the church and confronted the attacker, holding him there at gunpoint until police arrived. With one dead and seven wounded, the attack could have been much worse.
Arlington, Texas, May 3, 2017 (Dallas Morning News)
This attack was stopped before more than one person was killed. Yet, the police spokesman made it clear that the concealed handgun permit holder “prevented further loss of life” and that the killer was “shooting at the front door” at the customers who were fleeing the restaurant. The headline on the Dallas Morning News story was: “‘Hero’ stopped mass murder by crazed bar patron who was armed to the teeth, police say.” Emphasis added in quotes below
Townville Elementary School, September 28, 2016 (FBI report on active shooter incidents for 2016-17)
From the FBI report on page 5.
On September 28, 2016, at 1:45 p.m., Jesse Dewitt Osborne, 14, armed with a handgun, allegedly began shooting at the Townville Elementary School playground in Townville, South Carolina. Prior to theshooting, the shooter, a former student, killed his father at their home. Two people were killed, includingone student; three were wounded, one teacher and two students. A volunteer firefighter, who possessed a valid firearms permit, restrained the shooter until law enforcement officers arrived and apprehended him.
Lyman, South Carolina, June 30, 2016 (Fox Carolina)
Just a couple of weeks after the Orlando massacre, man, 32-year-old Jody Ray Thompson, started shooting at others at another nightclub. Fortunately, unlike Florida, permitted concealed handguns were allowed in bars in South Carolina. Before he could shoot a fourth person, the permit holder was able to shoot back, wounding Thompson in the leg.
The list goes on and the reason you never heard about them is because the media wants bodies not heroes.
https://crimeresearch.org/2019/05/uber-driver-in-chicago-stops-mass-public-shooting/