What is the caliber of the ar15 rifle?

cal. 223 or 5.56

The AR15 is a 5.56x45 rifle. Incidentally, it is also capable of chambering and firing the .223 Remington, which is similar in dimension, but the two are not identical. That's the norm. AR15 rifles can be had in a number of calibres nowadays, to include 6.8x43 Remington SPC, 6.5 Grendel, .458 SOCOM, .50 Beowulf, 5.45x39, 7.62x39, .204 Ruger, 9x19 Parabellum, .40 S&W, .45 ACP, 6x45, .450 Bushmaster, and others.