Many shooters will assert there is no difference between .223 Remington and 5.56x45mm NATO and that you can safely fire both from the same rifle. Others will contest that assertion, “The two ammo types are different!” But which one is right? Who is correct? Although the external dimensions of...
5.56 has a slightly larger powder charge, and the 5.56 chamber has a 1/8 inch longer chamber before the start of the rifling in the barrel, which allows the higher charge round to fire safely.
I bought a 5.56 rifle last year and the instructions for break in were to use 5.56 for the break in, so that it will cycle correctly and break in correctly. After that, I can use .223, just adjust the gas valve.
Not sure if your rifle has adjustable gas, or exactly what the break in instructions are. I don't see anything in the manual posted on Ruger's website. That is worth looking for. But the manual does say it can use both rounds.
In general, I would think that running a couple hundred rounds of good quality 5.56 would be the best approach. Try some .223 later on after everything is working smoothly.