The velocity change depends on the cartridge design and powder used...for example, I did some testing when the .270 and 7mm WSM came out, I compared the .270win, .300WM, and 7mmRem mag with the corresponding WSM's...The WSM's home should be in compact rifles...why? Because when I chopped the bbls down to 20", the WSM's with the only loads available at the time BARELY dropped velocities, while the long action cartridges started falling off quickly. The .308 looses less velocity than the .30-06 too. Basically, a lot of cartridges are chambered in bbls longer than they need for efficient combustion, but since they're a common diameter, there's no sense in making 15 different .308" bbls just because you produce 15 different .308" cartridges.

Bbl length changes can have huge impacts on velocity, which is clear by simply checking out the numbers on ANY cowboy action shooter's ammo....I roll ONE load of ammo for my CAS guns, to be used in both my Ruger Vaqueros (5.5" bbl) and my Marlin 1894 (20")...My load chrony's at an average of 867fps from the revolvers, but it's 1332fps average for my rifle.

As for the accuracy, that's a different story. There are a lot more things to consider than just the length that the bullet contacts the rifling. Changing the length of a rifle changes the weight, which changes it's inherent accuracy...there are a lot of things to consider for this loss of weight and shift of center of gravity.

First off, the rifle is going to recoil more, which effects the bullet about as much as it effects you, your "bullet contact" flip is going to be worse, because the rifle is pushed upward further and sooner since it weighs less towards the muzzle. It also shifts the center of gravity, or the balance point of your rifle rearward, which tends to change how well people can stabilize their rifles. The weight also changes how well you can hold still, It's hard to move a heavy rifle, so little body wiggles are dampened in a heavy rifle, while they're exaggerated in a light rifle.

If you're using open sights, it also changes the sight radius. Typically, the longer the sight radius, the more precise you can be with your shot placement...the geometry of this is REALLY simple, but it takes a while to explain, I'll elaborate if you're interested.

The effect on accuracy is really determined by the stability of the bullet as it leaves the muzzle...if it's stable at 4" (like a handgun), then you might not notice a change from 20" to 22" or from 16" to 26", in fact, you might even notice WORSENED accuracy with the longer bbl. BUT, if it's only getting stabilized at 21", and you cut your 22" bbl down to 20", you're going to have to find a new load, or a new bbl.