Without a mill, you could hacksaw a slot in the side of the bolt and use a thin extractor. The AK has a big military-style extractor designed to positively yank dirty or damaged cases out of the chamber. Many modern hunting rifles use much thinner extractors, basically pieces of sheet metal. They do this to provide more support for the case head for high pressure cartridges. Might not be a bad idea for a blowback, particularly since the extractor is only to remove an unfired cartridge from the chamber; the blowback spits the fired case out.
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