(snip) more seriously, what exactly were the advantages of the needle gun over what they were using at the time? is this an innovation of smokeless powder caliber?
I'm not sure, but I believe the OP was referring to the Dreyse Needle Gun, which was the first bolt-action rifle, & IIRC, the first general-issue breech-loader adopted by a major military, when it was adopted by the Prussian Army in 1841. It was known as the needle gun, because it used a long, needle-like firing pin to get at a percussion cap that was located between the powder charge & the bullet in a paper cartridge. It had a number of drawbacks, including poor gas sealing of the breech, poor range compared to other contemporary rifles, frequently breaking the firing pin which was a PITA to replace, especially on the battlefield, and rapid fouling of the barrel. However, compared to the muzzle-loading muskets & rifle-muskets that were almost universal when it was first adopted, the Needle Gun allowed for a much greater rate of fire, soldiers to be able to easily fire & load while prone, and helped drive the shift to more open-order tactics as opposed to the traditional linear and column formations.
It was already obsolescent by the time of the Franco-Prussian War in comparison to more recent breech-loaders, and was outclassed by French Chassepot rifles in that conflict, and it was replaced right after the Franco-Prussian War by the Mauser Gewehr 71.
In a delayed ACW scenario, the only reason I'd see for anyone to be using Needle Guns would be to equip second & third line units in face of an arms shortage that they tried to remedy by buying anything they could find in Europe, as I'd imagine that most troops in an 1880s-90s ACW would be using one of the versions of the .45-70 trapdoor Springfield or its equivalent TTL, breech-loader conversions of old rifle-muskets such as the Allin conversions of the M1861, and possibly modern bolt-action repeaters such as the Krag-Jorgensen or whatever the US Army would adopt as an alternative TTL (IIRC, designs from Springfield Armory, as well as the Lee-Metford, Mosin-Nagant, & Mauser Gew.91 were all in contention for that competition IOTL)
Foreign rifle purchases to fill gaps, assuming one was willing to deal with the complications of all sorts of extra cartridge types in the supply system would most likely be of things like Martini-Henrys, Russian Berdans, Gras rifles, Mauser Gew.71s, & whatever the Austrians were cranking out at this time.