You'll be aware that you aren't allowed to focus or otherwise adjust your scope during an hft shoot, so a compromise setting is needed; with that in mind a 42mm or so objective is the largest you would want as that increases the depth of field and therefore the usable range of the scope. If you have a 56mm objective this limits the range of non blurred objects too much.
This is why many shooters look hard for a good scope with 32mm objective or less. Equally in order to maximise d.o.f. it's not practical to shoot hft at more than 12 x mag.
It's the Indian not the Arrow