The famous one lots of people want to ban: The AR-15. This is the weapon Adam Lanza used in the Newtown school shooting. It's the popular civilian knockoff of the U.S. military's M-16. Here's a GIF made from a cutaway animation on Bushmaster's site. For you gun n00bs, this is what semi-automatic means -- you pull the trigger, it shoots a round, and automatically reloads:

The AR-15 is very customizable, as you can see in this GIF, posted on a Pennsylvania Firearm Owners Association message board.

For the most part all those add-ons are primarily cosmetic (folding stocks, bayonet mounts, pistol grips, etc.), which is why some AR-15 enthusiasts like to call them Barbie Dolls for Men. But one of the ways the 1994 assault weapons ban determined whether a semi-automatic gun was an "assault weapon" or not was if it had a detachable magazine and could take two or more of these add-ons. (It also outright banned certain models, like the Uzi and TEC-9.) As the Violence Policy Center's Tom Diaz explained to WHYY's Terry Gross, that created a huge loophole in the ban that gun manufacturers could take advantage of:

The requirement that you have at least two of those meant that gun manufactures could say, "Aha, we can keep the ability to take the high capacity magazine and just knock off the rest of these bells and whistles [and] we still have essentially the same gun, ... but it's now federally legal." And that's what Bushmaster figured out. They actually rose to prominence after the 1994 semi-automatic assault weapons ban because they took off all the truly irrelevant bells and whistles and just produced a basic gun.

This emphasis on the gun cosmetics has also opened up a tactic for the NRA to attack gun bans: it's racist to ban guns because of how they look. Former NRA president Marion Hammer said said on an NRA-produced news show last week, "Well, you know, banning people and things because of the way they look went out a long time ago. But here they are again. The color of a gun. The way it looks. It’s just bad politics."

However, some customizations of the AR-15 do more than just make it look cool. As Slate's Justin Peters explained this week, there's even a simple and legal add-on that allows the AR-15 to fire 900 rounds a minute. Slide Fire Solutions makes a replacement rifle stock, the SSAR-15, that makes it possible to bump fire an AR-15 from your shoulder. What does all that mean? Bump firing is a way of firing faster by using the weapon's recoil, but it usually requires holding the weapon at your hip. (Still from an instructional video unearthed by Peters at right.) That makes it harder to shoot your target, because, despite what we've all learned from gangster movies, holding a gun the cool way sacrifices accuracy.

But the SSAR-15 lets you to simulate the fully automatic setting while holding the weapon at your shoulder, so you can aim and correct. It's so easy, even a grandma can do it, as a Slide Fire Solutions video shows. Granny is simulating the M-16's burst setting, which shoots three rounds at once.