Will it screw onto the barrel of my Radical firearms 16" 300 blackout AR upper and how much does it add to the barrel length? — 2 Answers

This is used to make large frame LR308 barrels. It is not for small frame barrels like 300 BLK. When a blank is made for an AR, the chamber is cut on the back end of the blank and the outside threaded for this piece to screw onto. A bolt will be used to lock into the locking lugs with this receiver extension being screwed down. Once it locks up and headspace is set at minimum, the receiver extension is torqued down to 100 ft/lbs. and the receiver extension is pinned in place. That pin is the pin that slots into the upper receiver when you install a barrel to an upper. The gas port is then drilled on the opposite side from the feed ramps, whereever that may be. For this reason, a barrel extension determines headspace and can not be swapped out after the gas port is drilled. It does not add any length to your barrel, it is the part of your barrel the bolt lugs lock into.