A sabot-retaining shotshell cartridge and firing assembly including a barrel or barrel insert for firing the sabot-retaining shotshell cartridge. The shotshell cartridge includes a cylindrical hull, a sub-caliber projectile, a propellant charge, and a sabot. A central pathway or thin membrane is defined within the base of the sabot between the sub-caliber projectile and propellant charge. The barrel or barrel insert for firing the cartridge includes a transitional area between chamber and sub-caliber rifled bore to retain sabot in the cartridge. Upon cartridge firing, propellant gases flow through central portion of sabot base propelling projectile out of the sabot and into sub-caliber rifled bore.