of a land and groove
Photograph No. 7, Section D. Etched. x100
(90º cw; cropped, lower horizontals brought into closer alignment)
illustrating “Metallographic Examinations / Tests of trepanned rings from 12-inch gun” in Report of the tests of metals and other materials made with the United States Testing Machine at Watertown Arsenal, Massachusetts, during the Fiscal year ended June 30, 1913 (1913)
same (Harvard) copy and scan (less useful) at google books
is shown the hard layer and a crack developed at the junction of a land and groove...
is shown as almost structureless...
horizon; rounds; surface erosion; three theories seem plausible; Watertown Arsenal