In addition to the difficulties associated with transporting mortars (coehorns excepted) the safety issue comes up. More specifically, the NPS will not allow chambered pieces to be fired - either blank or live - at NPS sites. Mortars fall into this category. Despite the fact that with many pieces, 18th century 5.5 inch howitzers, for example, it is very easy to see if there is any fouling or embers in the breech. The concern is that because the gunner has to place his/her hands inside the bore, an accidental discharge would be catastrophic. The NSSA has specific rules/procedures, but even as good as those are, in our litigious society, they're apparently not good enough. Events on non-public property may/not fall under this prohibition.