Same way as rifle barrels:
- Wrap 45 caliber brush with strung-out copper* ChoreBoy from supermarket.
- Wrap enough that it's hard (but still possible) to get it through the barrel.
- Dip assembly in "lead remover" (or any solvent for that matter) and run it through the barrel 2-3 times.
- Wait 5 minutes and do it again, then dry patch out.
You are done.
* (Take magnet to the store to cull out and steel-core "copperized" ChoreBoy)