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I would consult Lilja about their capabilities.
They provide deep hole drilling services for more industries than standard gun barrels. Probably have the tools for the job to make you a 20mm barrel blank. I would call them knowing the twist rate you want and how much you are willing to shell out for the project.
I think your best bet is to call up various major barrel manufacturers and figure out which ones are capable of doing the project within your budget.
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