Ok, now you guys have me thinking.
Take a single shot rifle like the Handi, Drill and tap a hole in the bottom of the barrel at about the 4 inch from chamber point where max pressure happens.
Mount a second barrel of about 12 ga size (closed at both ends) to the rifle barrel and connect the two via a tube and valve.
The pressure impulse would be modulated by the expansion chamber but would still apply force to the bullet at a slower pressure but for a longer duration. Allowing factory ammo to be subsonic with low muzzle blast or supersonic normal with the turning of a valve.
Just a thought,
John