Lead melts at 621 deg. F. But then you've got liquid lead in the barrel, and maybe the internals.
Do not use heat. More than likely the barrel is 4140 or something close. At little as 500 degrees can affect the heat treat and aneal it, making that part of the barrel softer.
You do not want a barrel with an uneven heat treat.
Just get a proper rifle cleaning rod. Strip down the rifle completely and use the muzzle protector the rod comes with. Tap them out using the palm of your hand and be patient.
Were you using aguila colibris? Those barely have enough power to exit a rifle barrel let alone damage one. Aguila makes a super colibri you can use for rifles. However they don't hold a candle to subsonic rounds in terms of accuracy, power or penetration.
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