Reporter: They came in cases of 40, 40, 60 -- even 120 rounds. Maximum fire power packed into high capacity magazines. A lot more fire power. Reporter: When strapped into a pistol or semiautomatic... See More
Reporter: They came in cases of 40, 40, 60 -- even 120 rounds. Maximum fire power packed into high capacity magazines. A lot more fire power. Reporter: When strapped into a pistol or semiautomatic rifle, a shooter can fire until it's empty. Then, the damage can be devastating. And this is why the police chief in baltimore county, maryland, wants to outlaw all but the smallest of these magazines. Right here, this is it? Yes, sir. Reporter: The fewer the bullets, the more often the shooter has to stop firing, eject the empty cartridge and reload another one. And that window, why is that crucial? Folks that are being attacks have time to react. Reporter: How much time do you have in that window? We put a clock to it. An expert like this police officer can switch magazines in less than two seconds. We try it ourselves. Our fastest time, 4.2 seconds. Four seconds. Enough time for someone to flee the room or tackle the shooter. In this 2007 police video from texas, a suspect fires 11 times at law enforcement. When he 'em times his magazine, he dives back into his truck. That gives police time to open fire. It happened in arizona, during the attack on gabby giffords. Jared loughner was wrestled down when he stopped shooting to reload his pistol. And then, there was that school shootings right here in maryland, earlier this year. The suspect in this case tried to reload the weapon. We had a teacher tackle him during that process. Reporter: A teacher got him? A teacher. A brave teacher. Reporter: Last week, at sandy hook elementary, police believe adam lanza was armed with high whatty magazines. He didn't have to stop to reload until he had fired at least 30 times. Any reason why you think the general public should have a high capacity magazine? No. No place for it. Reporter: When he went to washington today, chief johnson met with vice president biden as he continues pushing for that ban. Meanwhile, diane, I checked with various gun shops around the country and those high capacity magazines, they're flying off the shelves. Some stores tell me they're sold out.
This transcript has been automatically generated and may not be 100% accurate.