How would you explain “gun-broke horses” here? We did not take more than a few steps before we heard the whine of incoming mortars. A bright sound like the sky had become a boiling kettle. We looked at each other, Murph and I, dumbly staring into our own infinity made up of fractions upon fractions of seconds. For one brief incalculable moment we were not brave or afraid. Neither spoke nor moved. Welled eye abuts welled eye, a look between gun-broke horses. How would you explain “gun-broke horses” in the last sentence? Thanks!