old thread, but i was compelled to post in it.
i know a bit about ruger rifles, and have worked in the factory.
what your talking about isnt a '2 shot" burst, its called "release fire". while this isnt full auto, its about as close as your going to get . i spoke to BATFE in length about this after reading this post. while in theroy it may be legal,do not get caught by BATFE shooting it. the one thing about doing this is that, if the trigger is pulled back slowly, it doesnt fire in a 2 shot mode, IT FIRES IN A FULLY AUTO mode in a condition called "slamfire", and is COMPLETELY AND UDDERLY UNSAFE, and a great way to either loose an eye, or to completely destroy your rifle.
this thread is one of the main reasons ruger decided to redesign there mini series rifles. if you dont have an older mini 14 or 30, youd better get one as there going to be few and far between in the near future, thanks to the guy who brought up this "2 shot burst" thread.