The old Douglas barrels were cold drawn to octagonal shape. This left very high residual stresses, tensile on the outside and compression in the bore.
I once carefully cut a slice of D. barrel, with water cooled abrasive saw. Measured the distance across flats exactly with micrometer (nominal 7/8" barrel). Then with same water cooled saw cut a slit in the ring I had cut. It sprung open considerably, from residual stress
Recall that Golden Age Arms used to have Douglas stress relieve their barrels, which would be done about 1100 - 1200F for a couple of hours.
Free-machining bar is normally hot rolled sligthly oversize, descaled, then cold drawn to meet
out-of-round tolerances fine enough to be held in a collet. That leaves the bar with some amount of residual stress in it. Those stresses will cause distortion if more metal is machined from one side than another.
Machining itself does add stress to the metal. Still, I suspect that most of the distortion one finds from machining mild steel comes from that steel being cold drawn, "cold rolled", without a final stress relief by the steel mill. Which is normal mill practice.
An accurate rifle barrel requires that the blank must be stress relieved at some point. May I minimize dispute by suggesting that many will acknowledge this to be true for modern cartridge rifles?
In Ancient Times the prints for GE gas turbine parts machined of nickel alloy always said something like "rough machine, then stress relieve/heat treat, then final machine"
