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Regards the TIG brightness, mask off everything that's black with shellac or wax or whatever and suspend the bolt in a sealed container with muriatic acid. The fumes will turn the stainless very dark grey.

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Probably not what you want to hear Lancaster, but being a 'non standard rifle'; perhaps a Cerakote finish on the bolt / bolt handle will alleviate that look?
The colours that they have are quite unique - perhaps 'Blue Titanium"?
https://wepleadthe2nd.com/cerakote-color-guide/