German 7,9x57 "S K 15 4"

So this one is just "K’. What was copper % before 67%? Why was that important to show copper content at 67 level but not before?

S67 was the 67/33 alloy used generally up to WWI, mostly for Rifle used, where the time lag between firing and case expansion/ afterpressure contraction was not a problem, even with the fastest manipulation of the bolt in a hot rifle ( rifle cooled etc during clip reloads etc)
With WWI, and later in 1920s Germany switched to S*, indicating 72/28 alloy, which was much better( more malleable) for high rates of fire in MGs of both WWI design and those of the 30s and 40s, with higher rates of fire.
There has been a more extensive explanation of the phenomenon in other IAA and Gunboards Posts.

“S” Indicated the Case design used in “S” Patrone…different from the case design of the “J” ( 1888) cartridge.

Initially the “K munition” was restricted ( by special Befehl) to shooting at loophole plates in opposing trenches due to its scarcity and cost. ( 1915) as production increased and per round cost reduced, SmK was issued for use in aircraft and ground MG, still in a reduced ratio.
( 1 in 5 or 1 in 10).

Doc AV

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Thanks, Doc,
For somebody like me, who does not know what “loophole plate” is, here is a picture Trench Loopholes, Le Linge | WW1 Revisited

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No, the brass alloy was 72% of copper and 28% of zinc up to 1915, and current 1915, they switched to an alloy made with only 67% of copper and 33% of zinc due to the rarity of copper because of the british’s naval embargo on germany. They switched back to the original brass alloy in the twenties

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Fully correct!