To summarize the research of John Houchins, Raimey, PeteM, and others regarding fluid steel barrels used by Hunter Arms:
Sanderson Bros. Steel Co. started their 'Syracuse Works' in 1876, and supplied Nitro and Crown fluid steel barrels to Hunter Arms starting in 1895, Armor in 1898, and Royal in 1901. Sanderson used the brand name "Sanderson Bros. & Co."
1906 12g 00. 'Armour' steel barrels 'P' is likely the Pittsburgh works

'C' possibly Cumberland works, Alleghany County, Maryland

Halcomb Steel Co. opened in 1905, and L.C. was on the Board of Directors. Halcomb supplied London steel for 0 grades and Royal steel for the hammer guns starting in 1907. Crucible Steel Co. bought Sanderson Bros in 1901, and Halcomb in 1911. "Crown" was the brand name of the Crown and Cumberland Steel Co., Allegany County, Maryland.
Crucible possibly obtained barrels from as yet unidentified suppliers like 'TD'

and 'W & R'

At least in the 30s, Crucible purchased tubes from
Jean-Baptiste Delcour-Dupont of Nessonvaux

and
Laurent Lochet-Habran
Damascus tube makers and suppliers are even more interesting
http://www.picturetrail.com/sfx/album/view/17116222Barrel marks to date:
EH - ?
Ernest Heuse-LemoineW & R
RIS & Co.
H.R.F. -
Heuse-Riga FilsO. C. & J.
WR - ? William Read
BD - ?
Bauduin Doyen of Nessonvaux