I doubt if it has a liner. It was likely just bored out to the larger caliber and then rifled. This was a common procedure back in the days after WWII. Lots of 30/06 bolt guns had the barrels reamed out to 338-06 and 35 Whelen. You don't see much of this any more. Sometimes I wish I had a deep boring and rifling setup. I could take all these shot out .22 CF barrels I have laying around the shop and bore them up to 6MM or .257 - get some beneficial use out of them.