Bramble:
Chrome plating can be easily stripped out of shotgun bores by plugging the muzzles, then filling with a 25% sodium hydroxide solution. A naked wire is suspended in the barrel and a 250 milli-amp current run through it.

PS:
I just found this link, which gives a more reliable recipe but says the fumes are toxic (knew that) and that the resulting solution must be disposed of as toxic waste (didn't know that!).

http://www.nmfrc.org/crarchive/jun02b.cfm