...the steel tube. Otherwise the much greater rate of expansion/contraction with temperature of the stock brass barrel would fatigue and fracture the solder joint.
Of course, you could forego the direct bond between barrel and tube. But then how much of the original gun's design and tradition would be left?
Of course, I guess that's just the old "slippery slope" all modders face. How much "modd'ing" is possible before - piece by piece - for all practical purposes - the original gun is simply gone?