Aztec Fire — Gary Jennings

The Aztec uprising in 1810 left young Juan Martez orphaned in a savage land. Like many native Aztecs, Juan was sentenced to hang by the Spanish. But his life is spared when a Spanish gunmaker learns of his skills with gunpowder and buys him to make munitions.
Keeping his Aztec heritage low key, Juan becomes the finest gunmaker in the colony--and the best shot. But both talents are kept secret--if the Spanish knew of his skills, he would surely be killed. Juan secretly works the Mexican underground, running guns and black powder to guerrilla forces fighting for independence. Along the way he falls in love with Maria Volza, a fiery revolutionary stirring up the masses, but she rejects him as fallen under the Spanish boot.
But Juan's destiny is to lead the Mexicans against Spanish rule, and when Maria discovers the life Juan has hidden so well, a battle is ignited and the revolution begins.