Except .177 is NOT "the most accurate calibre" because no 'calibre' is more accurate than any other,
.177 is simply advantageous to shoot because the flatter trajectory helps cover minor range estimation errors.
& frankly, if you "don't see the point" of FAC-air, you obviously don't know what you're talking about there's far more to it than just outright range.
Oh & "Rim or HMR" HMR is Rimfire