I have several 26" barreled rifles in calibers that "don't need" that barrel length: 308, 30-06, and 270. They are all quieter than 24" rifles in the same calibers which are quieter than 22" rifles, which are quieter than 20" rifles. As mathman said, it's the muzzle pressure. A 26" 308 is pretty pleasant and sounds like a popgun next to a 20" of the same caliber. I've built a few rifles with long light #1 barrels just to get the noise down for times when I can't run a suppressor. They also shoot much better offhand. I don't care about the free velocity, but it's there, though it's usually minor.
