All you need is a 308 bolt head, the firing pin length deals with placing a 223 short action bolt head onto a long action, which is not the case in your application. the parts are all the same Ejector, springs, extractor. I would use the 223 front baffle on a 6br, because it shortens the length of bolt travel. (This is what I do.) You will need to lengthen the ejector, or the BR case will not eject.
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You will need to change out the magazine box. The BR cartridge is hard to get to feed right out of a stagard feed magazine. The best solution is to use the Acme detachable magazine.

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