heavy cavalry engaging heavy cavalry

[Flavivs Aetivs]

Well there are a lot of similarities between the light crossbow (arcuballista) when used from horseback, and the black powder pistol when used from horseback. Both were able to be fired one handed, they were both rather slow to reload, and they both were not very accurate at longer ranges. They both also fired more flat and level than a bow.

But pistols are not relevant to the Roman era; an Arcuballista was not usually powerful enough to punch through chainmail, while a pistol would shatter it.

Heavy cavalry using the Arcuballista from Horseback would be plausible, but is unlikely.