A proposal from 1972 - a CH-47C helicopter with two 105mm XM-204 howitzers:
The XM204 used a ‘soft recoil’ system, where just before firing, the gun barrel assembly would be pushed forward by a pneumatic system pressurized with nitrogen gas. As the gun barrel assembly moved forward the gun fired, and recoil energy would have to overcome the forward momentum of the gun barrel and breech. This would disperse most of the recoil energy, allowing the XM204 to dispense with the usual trail associated with towed howitzers.