Aztec Fire (Aztec #5) by Gary Jennings, Robert Gleason, Junius Podrug

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.