Although I can't provide a citation, the fact that peak velocity on a .22 rf barrel is about 16-17" is pretty well established. It ain't rocket science.
As already noted, one reason for longer barrels, especially with traditional target rifles, is to maximize sight radius. Another reason, though not based on "more is always better", is offhand balance. There is a point at which too much weight out front becomes counterproductive.
There is also a theory that the slow time up the bore makes it desirable to get the bullet out ASAP when shooting offhand. That is why precision shooting with air rifles is so demanding, with follow through being critical.
In an effort to get the best of two worlds, some shooters have built rifles that are counterbored, leaving a short rifled section and a larger diameter smooth bored section out to the muzzle. I know one shooter who accomplished that by putting a short premium grade liner in a .22 target rifle, resulting in superb accuracy. Happens to be a vintage Winchester low wall Schuetzen rig, for offhand matches.
Paul
