Horses can be trained to tolerate loud, sudden sounds like gunfire but it takes time and by the end of its career you'll have a deaf horse. I think a four wheeled wagon with a couple of tonnes load capacity could definitely handle having a smaller sort of autocannon or a recoiless rifle fired from its bed but that would shake up even the best trained horses. The poor things would become nervous wrecks. I guess you could keep the horses in their yokes or harnesses but move them away from the wagon before firing.
Firing an M2 50 cal, Mark 19 or AGS-17 Plamya while stationary or moving shouldn't be a problem.
These are just my opinions because I have very limited experience with horses, wagons or heavy weapons.
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 "It is better to be feared than loved" - Nicolo Machiavelli
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