USMC a Machine Gun Mount would probably be easier to come up with. A guy in town has a Machine Gun Mount on his WWII Jeep I believe.
I would be impressed if Uncle Sugar put the Recoilless Rifle back in service. It is about like us not having an RPG of our own in inventory. If it does not cost at least $100,000 a shot, we do not need it. Cheap to shoot is not a bad plan in my opinion.
Bob R