You can not re-chamber a 223 Rem. to 204 Ruger. The 223 Rem. uses a .224 caliber bullet whereas the 204 Ruger uses a .204 caliber bullet. The use the same bolt head though and the same magazine. All you will need is a new barrel and the go and no-go gauges for 204 Ruger, along with the appropriate tools to swap a barrel (barrel nut wrench and an action vice).
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