How More Americans Carry Firearms Would Reduce Mass Shooting

Say it were true 25% of American citizens carried firearms. That's 1 in 4. Plug those hypothetical numbers into a recent shooting such as in California, or really any tragic event where a mass murderer has shot and killed 17 people and wounded another 8 people. Where 25 people were kill or wounded, the odds are that 6 of those 25 people had a gun on them. How does the number get up to 25? I think it's easy to say a mass murderer would be stopped by one of the people in the crowd far before the number got to 25. Not to mention, the likelihood that everyone in the area was killed or wounded is extremely low, suggesting that such a crowd could be 50 people, which would mean that 12-13 people who were not the psycho killer were carrying a firearm. Maybe the crowd is 100 people.
Of course the caveat here is that this situation concerning location would not apply in places such as schools or court houses, but we can also assume that if it were true 25% of Americans carried firearms on them it would be likely the culture would reflect this reality, such as school principals and certain teachers and definitely resource officers would be carrying guns, along with the assumption gun-free zones could totally be abandoned. This isn't far fetched when we accept the fact the 92% of mass shooting happen in gun-free zones. Psycho killers prefer gun-free zones. Making more gun-free zones and gun restrictions isn't the answer.
Now, extrapolate the possibilities of an America where 38% carried firearms with them, virtually all of them doing so for all the right reasons: safety and defense. How would this impact the concept of a mass shooting?
Discuss.
Of course the caveat here is that this situation concerning location would not apply in places such as schools or court houses, but we can also assume that if it were true 25% of Americans carried firearms on them it would be likely the culture would reflect this reality, such as school principals and certain teachers and definitely resource officers would be carrying guns, along with the assumption gun-free zones could totally be abandoned. This isn't far fetched when we accept the fact the 92% of mass shooting happen in gun-free zones. Psycho killers prefer gun-free zones. Making more gun-free zones and gun restrictions isn't the answer.
Now, extrapolate the possibilities of an America where 38% carried firearms with them, virtually all of them doing so for all the right reasons: safety and defense. How would this impact the concept of a mass shooting?
Discuss.