Caywood appears to stock kits with maple stocks. They will use pretty much whatever you want on a order, walnut, cherry, &c
With respect to proof testing, it is an ancient way of determining if there is a serious flaw in the barrel. Until recent generations it was the only way. I can guarantee you that by itself a proof test is, and has been, inadequate. One needs to magnetic particle inspect the steel bar or finished barrel, possibly eddy current test. I recall about 8 years ago touring Remington arms in NY, watching rifle barrels being mag particle inspected & some guy eddy current testing each shotgun barrel blank that they bought.
If anyone would like actual mechanical property data, and explanation thereof, for the screw stock most people use just email me. No responsible engineer with any integrity would specify this steel for a gun barrel, black powder or not.
I am well aware that no one reading this, unless there is an experienced mechanical engineer or steel metallurgist present, believes it.
The barrel makers & most shooters find my views amusing at best, except for those who are irritated because they wonder if maybe I might be right.
I trust you all notice that Green Mountain barrel company is known to use Gun Barrel Quality 1137Modified steel. Personally I will use either GM or Caywood barrels. I would prefer chrome-moly (4130, 4140) but settle for the GBQ 1137Mod. Just plain 1137 not a good idea.
Using 1137Mod GBQ means GM spends more hours to machine a barrel than if they had made it of 12L14.
Machining Time = $$$
Guess GM must do this just out of the goodness of their hearts? Don't you wonder why they ignore the potential cost savings of using screw stock?
Follow the Money Trail if you want to know why most barrel makers use 12L14.