Epoxy (ACRAGLAS for barrel lining), solder, or friction fit. Some gunsmiths I know use epoxy based bonding agents and swear by them. Others use the solder method. Soldering is a bit more complex to do.
With a 9mm barrel liner I used in the past, put a .070 walled steel tube over it that touched the Crosman long steel breech. The remaining breech covered section was simply the liner. Used 2 hour cure epoxy and then cleaned up all the excess with denatured alcohol on a rag. Ran a brush through the bore dipped in alcohol and followed with cleaning patches till very clean. Re-crowned the muzzle, then blued the entire thing again.
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