Barrett, commander of the militia regiment in Concord, had beaten them to it and removed the supplies at the last minute.
General Gage feared that the revolutionaries would be able to field artillery and threaten his forces in Boston Harbor. He intended to send the Concord expedition to seize these pieces.
"Four brass Cannon, & Two Cohorns or Mortars (so call’d by the Peasantry) Conceal’d at Mr: B——, (Lately chose or appointed Minute Colo.) Suppos’d to be deposited in his Cellar.—The Calibre of these pieces of Ordnance is not exactly ascertained, but reported to be only Diminutive. Two pieces of Iron Ordnance (Suppos’d to be 4 or 6 pounders) are mounted, (On carriages said to be very indifferent) in the Courthouse & watch’d at Night, hitherto by a Slender Guard of Minute men.—"
Written by Lt. Gen. Thomas Gage, from the report of a spy, 9 March,1775J.L. Bell, ” What’s So Important about Barrett’s Farm?” September 27, 2008, Boston 1775, https://boston1775.blogspot.com/2008/09/whats-so-important-about-barretts-farm.html.