The 1842 barrels were heavier than the 69' cal muskets before them, with enough material for rifling after the minnies came out. It was THE rifled musket of it's day, more accurate when rifled and shooting the 730gr. Minnie with 70gr. powder, than-were the .58's it bolstered the numbers of, in the armories.
Sleeving back to .69 would not probably work due to the thickness of the barrel and liner. It could, however be sleeved to a smaller size & quite easily perhaps to say .50 and maybe .58?