It would be a lot cheaper to buy a 25/06 barrel and pay somebody to cut it shorter than it would be to have a gunsmith bore, chamber and rifle (~$225) your .223 to make it 25 cal. The action is not strong enough for the wide short magnums anyway, backthrust is the product of pressure and surface area. An infrequent poster here made a 300WSM and it didn't last 100 shots before the barrel was floppy loose.
Why not shoot varmints with the .223, and get a 7mm-08, 270, 30-30, 308 or 30-06 barrel for deer?