Maybe, I'll bite.
I'm doing some research now.
Which ammo is of higher quality, the 223 or 5.56?
Some guns are intended to use only .223 REM. There is a misconception that .223 REM and 5.56x45mm NATO are the same. They have essentially the same cartridge dimensions, but the 5.56 is loaded to a higher pressure and 5.56 barrels are machined a bit differently in the throat where the bullet first contacts the rifling.
That said, guns made for 5.56 can fire either .223 REM or 5.56x45mm NATO, though accuracy with the .223 REM cartridge may be less than expected. Firing a 5.56 cartridge from a gun intended to ONLY use .223 REM could result in an overpressurization condition that could cause damage, injury or death. The AR-556 will reliably fire either cartridge.
About $0.40 a round for 5.56 Winchester ammo. What's an ideal price per round?
I'm seeing that 5.56 has better quality assurance.
Quality should be fine between .223 REM and 5.56x45mm NATO.
$0.40/round is kind of expensive. Look at patriotmunitionsllc.com (EDIT: shit, their website is still being upgraded). Even their reloads are good quality. For hunting, I buy much more expensive ammo (~$0.80-$1.25/round) but for plinking, target shooting and screwing around, I buy the cheapest ammo I can as long as it IS NOT steel cased and NOT corrosive powder and primers. Wolf, Tula and WPA are examples of cheap ammo that uses either steel cases, corrosive powder, corrosive primers or all of the above. Don't buy that stuff. Federal makes a sub-brand called American Eagle that can be found pretty cheap. I try to look for $0.25-$0.35/round pricing.