Adam Clarke on Revelation 9:17

Breastplates of fire - jacinth, and brimstone - That is, red, blue, and yellow; the first is the color of fire, the second of jacinth, and the third of sulphur.

And the heads of the horses - Is this an allegorical description of great ordnance? Cannons, on the mouths of which horses' heads were formed, or the mouth of the cannon cast in that form? Fire, smoke, and brimstone, is a good allegorical representation of gunpowder. The Ottomans made great use of heavy artillery in their wars with the Greeks of the lower empire.