Last night, The Daily Show with Trevor Noah tried to test the theory that mass shootings can be stopped by just "one good guy with a gun" (as the NRA likes to put on t-shirts). They did this by making correspondent Jordan Klepper get a concealed weapons permit. They then tested his ability to take down a shooter by putting him in a law enforcement-approved school shooting simulation. Klepper—who now has a concealed weapons permit that's valid in thirty states!—didn't do so hot. In fact, he ended up accidentally shooting a civilian and was mistaken as the "bad guy" by law enforcement.

http://www.cc.com/video-clips/w2bq3a/the-daily-show-with-trevor-noah-jordan-klepper--good-guy-with-a-gun-pt--2

You can watch part one of Klepper's report here. As Vox points out, multiple simulations have shown that most good guys with guns aren't able to stop mass shooters, and many of those "good guys" even make things worse. 

Also, NRA? Those t-shirts of yours are ugly. I know, this is besides the point, but come on:

Advertising
The only thing that stops a bad guy with a gun is a good guy with a gun.
This looks like something you buy after riding Splash Mountain.
NRAstore.com
Sources: Comedy Central