Patrick Ferguson was a British inventor and Army officer who developed a breechloading flintlock rifle in the 1770s (his patent was granted in 1776). He impressed British Army ordnance officials with a remarkable demonstration of the gun's speed and reliability, and was granted permission to organize an experimental unit of 100 marksmen armed with his rifles to fight in the American colonies. They first saw action at Brandywine, with indifferent results (100 men out of 30,000 redcoats would be hard-pressed to dramatically impact the outcome of a battle no matter how advanced their weapons). Ferguson himself was seriously wounded in the battle, and the unit was disbanded while he convalesced, never to be reformed.
The Ferguson rifle was not the first breechloading flintlock, but it was the first that was made to military standards and formally used in combat. The major innovation of Ferguson's was to machine his breech threads so that a single revolution of the breech would open it enough to reload (instead of requiring multiple revolutions). A rate of fire of 6 shots per minute or better was easily possible for a well-drilled shooter, and this from a weapon with the accuracy of a rifle. Other weapons at the time required choosing between the accuracy of rifling or the loading speed of a smoothbore. The Ferguson offered both - actually being faster than a smoothbore to reload, and allowing that operation to be done prone, behind cover to boot.
So why did the Ferguson disappear from use after Brandywine? The most immediate reason was the death of Ferguson himself in 1780 at the Battle of King's Mountain. His direct and personal advocacy was the driving force behind its use, and there was nobody to replace him in that role. In addition, the Ferguson rifles were necessarily much more time consuming and expensive to manufacture. Equipping the entire British Army with such weapons was simply not feasible financially.