The 4.2" mortar had a rifled barrel. IIRC, a crew in Sicily during WW II got a round into the open hatch of a kraut tank.
Probable errors are always going to be greater when either a mortar or howitzer fire a high angle mission because of the greater trajectories as opposed to a direct fire or low angle missions with with howitzers.
Any crew that put a 4.2" mortar round through the hatch of a tank was lucky, not accurate. I would bet that couldn't be duplicated in 100 rounds with the same survey, the same met., the same ammunition lot, and the same tube. On the other hand, I've seen crews put
successive 8" rounds (inert) through the window of a bunker on Signal Mountain at Ft. Sill. Mortars are useful when you don't have artillery, but for accuracy, true all-weather capability, quantity of steel on target over time, and availability of appropriate munitions, give me tube artillery.