It will work with any standard AR-15 upper receiver and standard AR-15 upper receivers are compatible with .300 Blackout.
Rise Armament does not provide dimensions - the "extended" refers to the width, not the length. It is compatible with the 5.56/.223 AR upper receivers. An AR10 in .308 would require a longer charging handle.
Mark H.
Yup. It fits perfectly.
The ar10/308 uses a longer charging handle, this one is only for ar15 platforms.