How is the liner held in? Solder, Epoxy, Bubble gum?
Once the liner is removed it sounds like you will need a rifle barrel with the bore and twist that you like and then turn it on a lathe to the outside dimension to fit the barrel that you want to reline.

A. J. Palik

If they used the red loctite, it may be nearly impossible to remove the liner without drilling it out.

Temperature at Which Loctite Breaks Down
Loctite threadlockers do not all “break down” at the same temperature — the limit depends on the specific grade, but most standard red grades (e.g., 271, 263) are rated up to about 300–360 °F, and high‑temperature grades can survive up to 450–650 °F before losing strength or becoming removable with heat 
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How Loctite Heat Resistance Works
Loctite products are anaerobic adhesives that cure in the absence of oxygen. Once cured, they retain strength well below their maximum service temperature, but above certain thresholds they soften enough to be removed with heat. This is why manufacturers recommend localized heating for disassembly.

Standard high‑strength red grades (e.g., Loctite 271, 263)

Max service temp: 300 °F (271) to 360 °F (263) 
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Removal: Henkel specifies heating the nut or bolt to ~500 °F to soften the adhesive enough to break loose 
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Practical breakdown: In service, these grades often start to lose holding strength around 325–400 °F and will release more easily once heated above ~500 °F 
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High‑temperature red grades (e.g., 266, 272)

Max service temp: up to 450 °F 
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STS Industrial

Removal: Still require ~500 °F for easy disassembly, but can withstand sustained operation at higher temps than standard red grades.

Ultra‑high‑temperature grades (e.g., 2422, 2620)

Max service temp: up to 650 °F 
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Removal: Similar heat‑softening behavior; removal still requires localized heating above ~500 °F.

Key Points
Breakdown ≠ melting — Loctite doesn’t “melt” like plastic; it softens and loses shear strength as temperature rises.

Service limit vs. removal limit — The “max service temp” is the highest continuous operating temperature; the “removal” temperature is higher, where the adhesive becomes soft enough to be loosened with heat.

Grade matters — Always check the product data sheet for the exact temperature range and removal instructions.

Practical tip — For standard red Loctite, expect it to start losing strength above ~325–400 °F and to be removable with heat above ~500 °F 
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Example: Loctite 271 (high strength red) is rated to 300 °F service, but will release with localized heating at ~500 °F 
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STS Industrial
. Loctite 266 (high temp red) is rated to 450 °F service, still requiring ~500 °F for removal 
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If you need a fastener to stay secure at high operating temperatures, choose a grade rated for that range and be aware that heat removal will still be required for disassembly.