What's wrong with the 204 Ruger?
I'm assuming this idea is from brainstorming on ways to save money?
The little bit of money you save with using 223 brass instead of 204 Ruger brass will all be ate up and then some when you pay to have the barrel rechambered. And there's no guarantee the factory barrel chamber is cut straight. If you own a bore scope, you will see a lot of bad chambers on factory rifles.
If you are really set on having a 20 Practical, you'd be better off finding a cheap Rem 700 action and throwing a decent barrel on it.
Wilson makes a really good shooting barrel that is very affordable (same barrels used on Cooper rifles).
Remington 700 actions
http://northlandshooterssupply.com/actions/remington/
Wilson 20 cal barrel, 1 in 10" twist, LV contour
https://raggedholebarrels.net/colle.../products/204-caliber-1-10-twist-light-varmit