With the AR series of rifles, it may be easier than you think to make one that is illegal. These guns are very popular because of the way that they can be configured and customized, but it's important to make sure you know exactly what you're legally allowed to do, or you could be facing charges.
For example, you can buy both AR pistols and AR rifles. This is an important distinction because they have to be registered properly, in the different categories. It's illegal to have a rifle with a barrel that is under 16 inches, but some pistol barrels are just 7.5 inches.
The issue here is that it's not hard to take a barrel off of one AR and put it on the other. In just a few minutes, you can do this at home. However, if you switch that short pistol barrel to the rifle body, you have now created a short barreled rifle (SBR) which is illegal for anyone without Class Three certification to own.
Another issue comes with flash hiders. You can buy these in different lengths, but many popular ones are 1.5 inches. If you have a rifle with a 14.5-inch barrel—too short to be legal—you may think adding the flash hider makes it legal. It can, but only if the hider is permanently fixed to the barrel. You can't just thread it into the end and then assume the gun is legal.
As you can see, with a gun that is so easy to customize, these are simple mistakes you could make. If you do, you'll need to know all of your legal options when facing charges in New York.
Source: Off The Grid News, "How Your Gun May Be Illegal And You Not Know It," Adam C, accessed Jan. 07, 2016