Bonny, many centerfire rifles ARE quite safe to reline to the original or another, smaller, caliber. It was a common practice back in the heyday of single shot rifles and still fairly common. It is the best way to restore a classic rifle with a shot-out or abused bore to close to original.
What ones are unsafe? I'm not sure of this but my general rule of (arthritic) thumb is first of all no rimless calibers. I'm SURE there are exceptions, but remember we're talkin' "thumb" here. Beyond that, I'd go mainly with the barrel OD/ID issue: is there gonna be enough "meat" in the barrel to hold the pressure and leave an adequate outer "sleeve" for the liner to set in. Are you counting threaded hole and dovetail depth????
The "thinnest" I've got is a relined Win 1885 "Lowall" in .38-40 with a #1 weight barrel. But I've seen the same gun relined to .44-40.
I WOULD NOT try to reline a .30-40 back to .30-40, but have seen a .303 relined back to .303, which I would also not attempt. But have seen several .30-30s.
Probably the greatest advantage of relining is that it preserves external barrel markings and allows you to NOT have to recut the various dovetails and other external profiles all over again on a new barrel blank; also keeps the original barrel's fit to the receiver, so no rethreading and figuring out how to keep everything squared up with the old receiver. This is particularly helpful in restoring vintage lever action repeaters, which have a LOT of cuts on their barrels to duplicate.
I've never heard of a liner failing (perhaps some of the gunsmiths here have, tho.).
