I too agree that for the interests of safety, *anything* hard being launched as a projectile and coming down vertically at anything close to terminal velocity is pretty much a non-starter on airsoft fields. I definitely wouldn't play on a field, with one of those in play, in any case.
However, *this* looks fun, if somewhat massive overkill in terms of a project.. i could see it being adapted as a much more economical mortar tube setup - and it'd be great if those nerf projectiles could be modified with an internal striker-plate of some sort to accomodate cap-gun type paper percussion charges (are those even available anymore? - i might be dating myself as a child of the 60-70's here) for the 'explosive' realism.
Airsoft howitzer big bore breach load - YouTube