It is not so much that they are "semi-auto", it is that they ARE NOT full auto.
The most popular configuration for a trigger on any firearm is that the activation is from the application of pressure. Yet, there are also configurations of "release triggers", whereas the firing mechanism is tripped upon the release of the pressure. Those are usually found on trap guns.
The binary trigger just combines the two configurations into one unit. Which of the two types would you write a law against so that you could say that the pull and then release of the trigger are a single activation that fires more than one round so that you can say it is full auto?
This is why the ATF can not legitimately prohibit bump-fire stocks. The firearm has to be pulled forward to activate a single function of the trigger, and has to be repeated for every round fired. This is not the same as with the xcelerator (sp? not going to look it up) stock that contains a spring in the stock that gets compressed under recoil and then pushes the firearm back forward, activating the trigger, and continues firing until the finger is removed from the trigger.
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