Savage uses a barrel nut, but doubt they were the first.

Does anyone know who first used a barrel nut ?

Hammer

Probably was first used by the military for ease of removing/replacing barrels on weapons that wore barrels out quickly, ie.: Fully automatic weapons.

The Dan Wesson revolvers used a barrel nut to hold the barrel in place also...it screwed the muzzle end to the barrel shroud and pressed the shroud against the frame to hold everything in place.

Many take-down systems use a nut.