You have to take "stress relief" literally. It doesn't mean "stress elimination". Thermal stress relief describes a sub-critical process (below the temperature that would cause internal granular changes) which removes some of the stresses trapped in the material that were caused by forging, rolling, welding,or other working, by heating it to a point where slip planes can occur in the structure of the material and allow relaxation of peak locked-in stresses first and more slowly and less effectively for lower stresses. It won't eliminate all stresses. I don't know much about the cryogenic treatments or their effect on stresses, if any. Also, there is a type of mechanical (vibratory) stress relief that I haven't seen to be effective.
Worth noting: if a material has been stressed past it's elastic limit, (beyond its yield strength), stress relief can not return it to its original shape. Example: a barrel bent by a mishap won't straighten by stress relief, or a button rifled barrel will not return to its original un-rifled state by stress relieving it. Thank goodness!
Bottom line finally; after stress relieving a barrel, some stresses remain trapped, and the material can and probably will creep and crawl measurably if enough material is removed (fluting), or the material is upset (pushed around and compacted) by button rifling. All the barrel makers mentioned have learned how and when to remove enough stresses to enable them to provide stable, straight, uniform, superbly accurate barrels without damaging the material they are made of. Let's hear it for the barrel makers. If it was easy, everybody would be doing it!
I hope I managed to say what I meant to, and that it helps. Tom
Texas State Rifle Association Life Member
NRA Endowment Life Member
A big fast bullet will beat a little fast bullet every time