AR-15 Pistol Rifling Twist

I recently built this AR-15 pistol from a virgin lower receiver that had never been assembled into a rifle. In reading the gun forums concerning AR pistols, one of the recurring themes is that only a fast 1:7" rifling twist should be used in the shortest common AR pistol barrel length of 7.5 inches, because that barely allows over one full rifling turn.
By that thinking, it should be impossible for the typical .38 Special revolver to shoot accurately. One turn in 18 inches is a common twist rate for .38 Special and .357 Magnum, and even a 6 inch barrel only provides one third of a turn to the projectile.
I bought my AR-15 pistol barrel with a 1:9 inch twist. So far, it seems to shoot just fine.