The first time I shoot a new gun, I use factory ammunition. That way if there is a problem with the gun, the manufacturer (or if it's a really bad problem, the manufacturer's attorney) can't blame it on my reloads.

After that, every round I shoot is one I have loaded myself.

Although I have had to use various powders over the years based on availability, my predominate load is 20.3 grains of IMR-4198 under a Hoernday 60 grain soft point or hollow point bullet producing a velocity out of 16 inch barrel of around 2800 fps. Based on Hornady Manual #4 and Hodgdon Manual #25, this should produce a chamber pressure below the 52,000 psi SAAMI max. for .223 Remington.

So, I guess my answer would be that I use .223 Remington in my AR.

ALSO PLEASE NOTE the manuals I rely upon for the load specified above were printed in the 1980's and later editions have LOWER maximum loads. Always load using data from current published reloading manuals.