Black powder is a relatively simple thing to put together, with the earliest recorded use by China in the 9th century, and plenty of ATLs in which this or that iron-age civilization chances upon it, like Robertp6165's "The Guns of the Tawantinsuya". Smokeless powder is a different animal, and requires fairly advanced notions of chemistry to develop. Hence this challenge: what is the earliest plausible date for someone to come up with a usable form of smokeless powder, and what would the consequences be?
I'm guessing it probably wouldn't be before the early 19th century at best, though in a TL like Thande's "Look to the West", chemistry seems to have progressed noticeably faster than in OTL in the second half of the 18th century. But even if we posit development around the 1840s (guncotton, an unstable early version of smokeless powder, was invented in 1846), it could have quite an impact for such events as the American Civil War and the Franco-Prussian War.