Yes, there definitely is a difference. My understanding is that the longer the barrel, the more powder actually gets burned, thus increasing bullet velocity. It makes physical sense that accuracy will also be positively affected by a longer barrel, since the rifling will have longer to get the bullet spinning correctly.

Barrel lengths do effect velcities, though often in unexpected ways. In testing some of my .357 Magnum handloads, some powders delivered higher velocities in a 4" barrel than in a 6" barrel. Usually, however, the opposite is true.

As to accuracy, a man can usually shoot more acurately with the longer barrels. This because the sight radius is longer. The longer sight radius affords better view of aiming errors, and is more forgiving.

Bob Wright