.223 to 22arc?

There's a ton of variables, existing barrel length, port size, buffer setup, springs, loads it's been tuned for.
New barrel length, port size, buffer setup, springs, loads you want to run.
You would have to specify all those things before anyone could help further. And said help may just be an educated guess anyway. Sometimes you just have to do it and see what happens.
How picky are you? If it goes bang are you happy? Are you OCD about finely optimizing every small detail of a build?
Like I said, it depends.
 
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I agree with LRRPF52, get a whole 22ARC upper that will be madeto all work together. You can keep the 223 upper in case you want to bang away with cheaper ammo.