A polymer-based cartridge for subsonic ammunition with a first end having a projectile disposed in a mouth, a shoulder forming a bottleneck cartridge; and at least a polymer wall between the first end and a second end opposite the first. An insert is joined to the second end, having an extraction rim and groove, a primer pocket communicating with a flash hole, and the flash hole communicating with a propellant chamber. A sleeve section has a metal or a metal alloy, extending from the insert toward the first end, and at least partially covered by the wall. The sleeve section and the wall form the propellant chamber having a thickness at least 1.25 times greater than a standard thickness of a wall of a standard cartridge. The propellant chamber between the mouth and the insert is unobstructed, and comprises a powder load having a load density greater than 40%.