Question on the development of the "Military secret" concept

First of all, if this belongs in chat, please move it.
I'm working on my 1870's timeline, and can't find out what sort of things might be miliary secrets at the time. Would a new fire control system's existence be known? Would it be secret when it reaches the fleet?
Would anyone be able to look up the formula for a new powder, or easily find out how good a new locomotive torpedo is?
How would it evolve by the 1890's? My timeline will be including a "War of 1897," when new gadgets get used in combat for the first time--how much will other parties suspect about the unprecedented long range fire control of one American battleship?
 
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