See if you can get the nipple out. Flush the barrel out in soapy water, then clean water, then shoot it some.
That might loosen it a little. Also, what if you put the barrel in a vise, heat it up around the breechplug, then try to remove the plug? Don't heat it very hot, you don't want to ruin the temper in the barrel. These are just ideas I have. They may or may not work.