There are other trigger modifications (suck as lightning links or drop in auto-sears) that have been considered to be illegal machine guns because they can be used to convert a standard semi-automatic weapon to full-auto fire.
The ATF currently does not consider a binary trigger (or a firearm with a binary trigger installed) to be a machine gun because by law a machine gun is considered “any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot without manual reloading, by a single function of the trigger.” This essentially means you have multiple rounds sent downrange per each pull or action of the trigger, so since a binary trigger uses two actions to fire two shots, this is not by definition a machine gun. Federally, these are legal. Some states/counties have more stringent laws. More and more pressure is being placed on regulating these devices and effectively setting the equivalent of a speed limit for firearms. This is similar to our previous discussion on bump stocks (click here to read that article). Just because it is legal does not mean it is the best option for self-defense. Make sure to check your state and local laws so you can make sure you in possession a legal attachment for your firearm.
Some of you may already know of Binary Triggers and even may have used one before, but let's refresh exactly on what it does.
firearmslegal.com
ARE THEY LEGAL IN YOUR STATE?
It is always highly encouraged to determine whether your state allows using binary triggers legally to avoid being charged with unlawful usage of such tools. Based on our research, a binary trigger is currently banned in North Dakota, Hawaii, Connecticut, New Jersey, Maryland, Washington, California, D.C., Iowa, New York, and Rhode Island, Florida.
Adding a binary trigger to almost any rifle, pistol, or carbine lets you increase the rate of fire. Before you pull the trigger, you want know, are binary triggers legal? We’ll guide you through the important things you need to know.
lundestudio.com
So no go her in NY but ok other places. It would be fun to try one but not worth the $200 to me, especially with ammo prices these days.