Some quotes from the wikipedia article on Minutemen:
"The need for efficient minuteman companies was illustrated by the Powder Alarm of 1774. Militia companies were called out to resist British troops, who were sent to capture ammunition stores. By the time the militia was ready, the British regulars had already captured the arms at Cambridge and Charlestown and returned to Boston."
"With a rising number of Minutemen they faced another problem: they didn’t have enough gun powder to support an army for long enough to win freedom. They needed powder badly."
"Two thousand militiamen marched to intimidate the judges and get them to leave. This was the first time the militia was used by the people to block the king's representatives from acting on royal orders and against popular opinion. Gage responded by preparing to march to collect munitions from the provincials. For 50 miles around Boston, militiamen were marching in response."
"The royal authorities in Boston had seen these increasing numbers of militia appearing and thought that if they sent a sizable force to Concord to seize munitions and stores there (which they considered the King's property since it was paid for to defend the colonies from the American Indian threat), the militia would not interfere. The events on April 19, 1775 proved them wrong when the mobilization fielded large numbers of minutemen to confront them at Concord that, with the arrival of the slower moving militia, rapidly outnumbered them and forced a strategic defeat on Colonel Smith forcing him back to Boston."
"Ammunition and supplies were in short supply and were constantly being seized by British patrols. As a precaution, these items were often hidden or left behind by minutemen in fields or wooded areas. Other popular concealment methods were to hide items underneath floorboards in houses and barns."