Something you might consider is the process used in launching modern rifle grenades, that is, instead of the traditional special cartridge (usually some type of blank), a normal ball round is used. The body of the grenade is designed to trap the bullet and absorb its kinetic energy along with energy from the escaping propellant gas.
As applied to a muzzle loading cannon type device, the line carrying piece would be cup shaped to trap the shot, and would either sit in a depression in the muzzle or, if the muzzle were of small enough diameter, sit surrounding the muzzle. The amount of overlap between the muzzle and the cup would determine how much energy from the powder gas was captured.