I agree with Stubble. In order to do this you will need to buy a new barrel and then pay a gunsmith to thread, crown, chamber, and set headspace. You will also need a new bolt since the .223 has a smaller case head. You will also need to purchase a new magazine well, follower and magazine spring because the .223 is a smaller diameter cartridge. Then you got the cost of finishing the barrel, either blasting or polishing the stainless and chrome moly and also blueing the chrome moly.

Add it all up.

New barrel - $200 chrome moly, $300 stainless
Labor fitting and chambering barrel - About $300
New bolt - $130
Mag well, spring and follower - $50
Finishing the barrel. $75 - $150

All in all you are going to spend $600 to $800 doing this little project.

You could get a new rifle in .223 and still have money for a scope.