Within Limits, Yes.

You can for example uses a. 223 in a .308 boltface or a .308 in a .535” Magnum.

Obviously you will replace the barrel .The extractor claw is replaced as well and you may find the ejector plunger is better removed as reliable ejection is not always possible. And your gunsmith should be up with the feeding requirements of Barnard actions

The extractors in a Barnard target action lie at the bottom of the bolt face when closed. When the bolt is open the extractor still sits below the waterline, so although on extraction the smaller case may drop a little it falls into the extractor. If the manipulation of the rifle is moderate the action may extract satisfactory. But in the usual target shooting, benchrest and in at least some stable hunting situations the smaller case will feed and extract satisfactorily.

For further information on extractor fitting please view this extractor fitting information page.