I have an old gun and am worried about possible barrel damage.

Tests have found that standard performance 24g steel loads did not cause any damage in thin-walled game guns, even after a thousand shots [1]. However, if you are unable to use steel then your main alternative is bismuth, which is suitable for both muzzle-loading and Damascus-barrelled guns.

Bismuth does cost more than both lead and steel but, ultimately, this is all about choices and securing the future of sustainable shooting.

[1] The Assessment of the Tolerance of Shotgun Chokes to Steel Shot – An Initial Study: Dr DF Allsop, Royal Military College of Science (1991).