Here in the UK most stalkers and many estates now use / require a silenced rifle. With a traditional combination gun or drilling there is no way you could screw on a moderator. But would taking an Einstecklauf and drilling a series of porting holes near the muzzle so that the gases can vent into the shot barrel and then back out work? Could be very useful on a 22rf or 22 cf fitted into a shot barrel.

Doable - recall talking to a certain Brit gun-smith who works in Germany. He said he had constructed a moderator that was permanently fitted over such a ported eistecklauf.

I would think that given the restrictions within the bore of your gun, such a thing could easily be constructed as an addition to a short insert unit!

I know that the short Krieghoff inserts in 22 and 22 Magnum are noisier fired inside a shotgun barrel than the same calibers fired from full length rifle barrels. It would be interesting to see if putting a ported and unrifled extension to the short insert, with some sort of cap at the shotgun muzzle would muffle the sounds. But that may constitute a silencer and in the US it would require registration and a fee.