Sir Joseph Whitworth's adaptation of Bessemer's principle of hydraulic pressure casting was patented in 1874. The primary advantage in the fluid compression process was uniformity, and the absence of porosity and voids.
The Mechanical and Other Properties of Iron and Steel in Connection with Their Chemical Composition, 1891, reported Whitworth Gun Steel as .30 - .42% carbon and .24 - .31% manganese.
For comparison, Vickers was .24 - .27% carbon and .22% manganese; Krupp .46 - .52% carbon and .07 - .13% manganese.
https://books.google.com/books?id=-c8xAAAAMAAJ&pg=PA196&dq Wm. Powell & Son used Whitworth for barrels in 1875. The first Purdey Pair Nos. 10614 & 10615 were delivered January 1, 1880 with the New Whitworth Fluid Pressed Steel. Lefever Arms Co. was the first U.S. maker to supply Whitworth steel for their Optimus in 1887. Parker Bros. used Whitworth for the first AAH Pigeon Gun in 1894 SN 79964 delivered to Capt. Du Bray. Hunter Arms first offered Whitworth on the Monogram, A2, and A3 in 1895.
