Apparently, that airgun barrel needs to be that long, because compressed air doesn't have the max pressure of a gunpowder charge. That's unfortunately a significant disadvantage, for airguns.
Also notice what they say: the 200 grain bullet is not spinning fast enough at 1:8" twist; if shortened, the bullet will also become lighter, so perhaps another barrel at 1:7" twist would better accomodate the 200 grain and even slightly heavier subsonics.
As for the hunting part; what would be the energy at the impact point?
A subsonic bullet for deer size targets might need about 220 - 240 grains of weight, to deliver what is normally considered enough energy to the target, even at close to medium ranges.
That's also the current weight limit of a .30 cal bullet, 230 -240 grain being considered a "heavy for caliber" bullet acording to current opinion.
So yes, the airguns are great but they have their limitations even when compared with subsonic gunpowder cartridges, and the much longer barrel lengths required for airguns could be somewhat annoying for hunting.
