At Gettysburg, the repeating rifle and the breech-loader.
Day 1, John Buford brilliantly deploys his cavalry in preparation for a prolonged defense in depth. The high rater of fire thrown out causes the advancing Confederates to pause and deploy in line of battle a number of times, losing critical time.
Here's the "First Shot" monument, erected by the guys who took it. Its on the main drag from Chambersburg (Hwy 30??) a couple of miles west of the battlefield proper.
"...if the gentlemen of Virginia shall send us a dozen of their sons, we would take great care in their education, instruct them in all we know, and make men of them." Canasatego 1744