The M231 had the most insane rpm.

There is a nice, very detailed write up with all the photos of it taken apart on arfcom, the open bolt striker is interesting.

That stock has been done in history on the AR a few different times.

there is a guy on ARF, RDTCU, who has some sweet stuff like that, google honeybadger and stock and AR15 and you will see a topic with his in it.

There are enough changes that it is a different weapon and should be studied to see what is the best tuning for it.