I have had two 20 gauge Flues guns color casehardened. In both cases I prepared them and Doug Turnbull did the hardening. I did have some minor movement on the first one that required hard fitting, the second didn't move at all. I have also straightened one out that Doug DIDN'T do, it was a mess but turned out fine in the end.
Warpage is almost a given with 20 gauge Flues guns due to being so thin. You have to go into it pretty much expecting that you will have to do some hard fitting in the end.
Best, Dan
