I've spoken with a number of Gunsmiths down under about re-barreling/having inserts put into existing double/triple barrel shotguns to convert them into rifled firearms, however every single gunsmith I've contacted has turned me down and it was everyone on the whole first and second page of googles search for gunsmiths in Australia.
The only one that would do it, would only do so for extremely light round (he mentioned .222 Rem) but said anything larger would be to dangerous to consider.
At first I was excited about the prospect upon its realization but after contacting the gunsmiths I went back to disheartened.
Short, adjustable insert barrels are a "very german solution", pioneered by Krieghoff between WW1 and WW2.
The first were made in 5,6x35R Vierling, nothing but a low pessure version of the .22 Hornet.
After the war, .22 long rifle and .22 Winchester Magnum became popular and were present in almost every drilling.
In the 1970s the bavarian gunsmith Zimmermann made the first full length, muzzle adjustable barrels in .22 Hornet, .222 Remington and 5,6x50R Magnum.
Today, Keller & Simmann, Keppeler, and some others dominate the market.
https://www.einstecklauf.de
https://www.keppeler-te.de/pages/produkte-products/jagd-hunting/einstecklaeufe-insert-barrels.php
They make barrels up to 9,3x74R an even .45-70 Government.
Most importend thing is a gun with a stabil, durabel break open action, …..and enough time & money.
Krieghoff does not make them any longer.
Funny things are possible....
…. a former Heym 5,6x50R Magnum/20 ga combination gun, converted to an 5,6x50R Magnum over/under double rifle…..
My dangerous game rifle for hunting fox, jackal and racoon…..
HWL