I wouldn't shorten the barrels. Unless you need a rifle for charging tigers when sitting on a howdah saddle on an elephant.

Are the bores stuffed in the existing barrels?Or just the ribs needing replacement? Well then just fix up the ribs.

Turning it into a .44 pistol cartridge, people will ask in the future, "why did a previous owner do this, to this rifle?"

IF the double rifle is already worthless, well then no loss.

What are the Danish requirements for roe deer?

As you can guess, I don't think much of the project. But its your rifle.