Here is the recent, and now locked, threadhttps://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=561800
The (probably pre-1920) Model 12 Nickel Steel barrel is non-standard (just slightly low nickel at 3.17%, standard is 3.25-3.75%; and slightly low manganese) AISI 2340 medium carbon low alloy steel.
C - .39% (.38-.43)
Mn - .65% (.7-.9)
Sulfur and Phosphorus both < .04%
Cr - .08%
Mo - <0.01%
Here's the WOW
Yield strength - 100,000 psi
Ultimate tensile strength - 124,000
% elongation - 19
I found an industrial standard tensile strength for 2340 of 94,275 psi so the barrel may have had some form of heat treatment?
Thanks again to Channing.
And BTW I've got a section of Lefever DS Dura Nitro barrel coming