PHILADELPHIA (WPVI) -- Anyone with a few hundred bucks can buy what's called a 'Ghost Gun' online. Within four hours, we built a semi-automatic firearm in the Action News Investigation office.And law enforcement now tell Action News these same guns are ending up in the hands of criminals on Philadelphia's streets.AK-47s, AR-15s, complete do-it-yourself kits for almost any kind of assault rifle or pistol, are peddled online, to anyone. No ID, background check, date of birth or any identifying information required.It cost Action News just $600, and five days later our nearly complete weapon arrived at our door.With the help of complete how-to instructions posted online and an Action News engineer, we drilled out the trigger chamber, called an 80 percent lower, and built a fully functional firearm.Former ATF Special Agent in Charge Sam Rabadi said it's these guns that pose one of the biggest risks to law enforcement."What I have here in front of me is an actual firearm - an AR-15," said Rabadi "It is absolutely dangerous. As a law enforcement officer, the last thing I want to do is go up against a criminal who has a firearm like this. It's essentially a civilian version of an M-16 which is a military rifle."Beyond being powerful, when used in a crime, Rabadi said it hampers an officer's ability to track who bought the weapon, or find clues as to how it got in a potentially dangerous criminals hands.But these 'Ghost Guns' are 100 percent legal."'Ghost Guns' is another fabricated term by the anti-gun bigots that want to try and demonize something," said Alexander Roubian, President of the New Jersey Second Amendment Society.Roubian said there are legitimate reasons these low cost firearms are desirable to gun enthusiasts and laymen alike."This is actually very liberating for people who want to build a firearm for their own self-defense or personal use that might be on a fixed income, on a low budget," said Roubian.Roubian took Action News to a gun range to test the weapon, to show us how the gun fires and to explain why he believes a gun without a serial number should be legal."There's never been an epidemic with criminals going and building their own firearms," said Roubian. "If criminals want to acquire firearms as they do now, they find ways of doing it."But Rabadi, who headed Philadelphia's Field Office of the ATF for the last five years, said there's good reason for Philadelphians to be troubled."They are being used in crimes, they're being recovered at crime scenes," said Rabadi. "These types of firearms are available in the city here."This is a concern for Mayor Jim Kenney."We lost 277 people last year to gun violence," said Kenney.And he expects those number to jump 10 percent more this year."We are going to continue to have the carnage that we have," he said.Rabadi, who says he's a strong supporter of the Second Amendment, said he just isn't a supporter of unserialized weapons ending up in the wrong hands."This is just another avenue that makes it much easier for a violent criminal to get their hands on a gun that is totally untraceable," said Rabadi.Rabadi said since the laws do allow for people to make homemade guns for their own protection, it's in the hands of congress to close the loopholes that are allowing untraceable guns to end up on our streets.