The most significant Chinese military advancement in medieval China, in terms of impact on world history, is probably not a single piece of equipment or technology, but the body of military skill and expertise in seigecraft. It was Chinese seigecraft that allowed the Mongols to roll over central, south and western Asia. Without Chinese seige engineers the Mongols could not have easily rolled up the urbanized civilizations of Euroasian heartlands, and probably would have been stopped well short of reaching Europe and Near East.
The importance of the 200 years of "Mongol peace" stretching from China to borders of Turkey, made possible by Chinese seige craft, to the subsequent development of European, Near East, Middle East and South Asian civilizations can't be overestimated. No Chinese seige craft, no Mongol Peace, no Marco Polo, much less European inclination to explore westwards to reach fabled wealth of "Cathay" and India, Discovery of America would probably be delayed by centuries. The great European atlantic cultures that gave rise to capitalism would remain backwaters. Italy and Turkish empire would remain the commerical heartlands of western world. The world of 18th and 19th century when the foundations of modernity is laid would instead probably look much more like the world of the 15th century.