For excessive leading I use an Ultrasonic cleaner, some distilled water and Dawn dishwashing soap. Works great on any small parts that will fit into the basket. They are readily available, not that expensive and have all kinds of practical uses around the house. Obviously be sure to throughly dry then lightly oil before reassembly. Good luck...
Edit: ok, just reread the OP. Obviously an ultrasonic won’t work for a fix barrel unless you have one large enough to fit the entire pistol. Sorry, 🙄