This NRA article shows several of the muskets used by the militia on 19 April. Also, there is this interesting John Robbins witness account that a British Regular Officer ordered the firing at Lexington.
https://www.americanrifleman.org/content/the-shot-heard-round-the-world-the-arms-events-of-april-19-1775/
"Being drawn up sometime before sunrise, on the green or common, and I being in the front rank, there suddenly appeared a number of the King's troops, about a thousand, as I thought, at the distance of about sixty or seventy yards from us, huzzaing, and on a quick pace towards us, with three officers in their front on horse back, and on full gallop towards us, the foremost of which cried, throw down your arms, ye villains, ye rebels, upon which said company dispersing, the foremost of the three officers ordered their men saying fire, by God, fire, at which moment we received a very heavy and close fire from them, at which instant, being wounded, I fell, and several of our men were shot dead."