Ok,
I'm with you now.
I should have read your post more carefully.
In the case of rifle barrels sleeved full length into shottie tubes, yes, the regulation will be more difficult.
I fail to see much benefit in building barrels this way for a double rifle.
IMO it is as much, or more work, than doing a monoblok setup.
The full length sleeving idea can look clubby, lack feel and balance, and regulating can turn into a nightmare.

Regarding the difference between .410 shotgun cartridge pressure and 303 rifle cartridge pressure, there is a BIG difference.

You need to ensure that the thickness of steel surrounding the cartridge chamber is more than enough before even thinking about any shotgun to rifle conversion.
IMO most 410 double barrel shotguns will be too small for safe conversion to 303 double rifle.