One of the more obscure weapons concepts, the RAVEN cannon:
For lighter weight vehicles, recoil limitations are overcome by incorporating the larger caliber rarefaction wave gun
technology while providing guided, stabilized LOS, course-corrected LOS, and beyond LOS accuracy"
Excellent overview of competing recoil management techniques from Dr. Eric Kathe, US Army RDECOM ARDEC’s Benet Labs, presentation at Armaments 2015.
Fire-out-of-battery always struck me as the most readily realizable.