Technically, you can still get an AR15 (or modify an existing AR15) to fire 7.62 rounds still as well. Plus, some AR15’s, if they are of high enough grade, can fire both the .223 and 5.56 NATO rounds since the exterior casement of the bullet is almost identical but the punch that each bullet has is different.
And, to simplify things a little bit for the non-gun people, an AR15 is just an M16 without the ability for automatic fire.