I AGREE BUT ITS A GENERAL TERM USED TO DESCRIBE A BALNK OR WHAT EVER MACHINED AS FAR AS THE LAW ALLOWS.I never considered doing an "80%" because of the time involved in EDMing the action. By the way, I hate the term "80%". It is either a firearm or it is not. There is no "80%" firearm.
THE EXTRENSION BE ATTACHED IN ABUNCH OF WAYS DEPENDIONG HOW IT WAS MADE.I had never thought about doing a barrel extension... It would not be THAT hard to do.... A barrel extension and or bolt head would not require a determination letter. The question would be how to join the barrel extension to the action... It could be silver soldered with proper use of heat paste but you would not want to weld 4140 without proper stress relieving and heat treating. Rivet to tubing like an AK???
I supose if there were interest... I would have to think about that...
Now look what you started.
MY INITAL THOUGHTS WERE SIMPLY THREAD THE OUT SIDE OF IT AN SCREW IT INTO A TUBE TREADED ON THE INSIDE. IF YOU WANTED YOU COULD RIVET IT OR PIN IT IN IF IT WAS LONG ENOUGH THAT WELDING WOULD NOT AFFECT THE LUG AREA YOU COULD WELD IT IN.
CAN 4140 PREHARDENED BE MACHINED WITH HSS TOOLING ????????