Taking a POD of before 1900 but after 1750, what's the absolute earliest that a fully automatic machine gun based on the same or similar principles of the Maxim gun (or any other possible automatic loading mechanism) can be invented and soon after adopted by major militaries, mass produced, and put into use in a major conflict?
How does this affect the 19th century geopolitical trajectory of all the different powers? How would it affect the development of military tactics in anticipation of this technology coming into use so early?
What are the possible wars that see major changes due to earlier use of the machine gun, or are there any butterflied conflicts or any other events?