Actually, the speed of sound in steel is something like 23,000fps, which is about 7-15 times faster than most bullets travel, so can and do effect the bullet while in the barrel (actually at the muzzle). As far as beddings effect on accuracy, it's mostly through consistancy that bedding makes a difference. Accuracy is synonymous with consistancy. Hypothetically, if everything happens exactly the same way every time a bullet is fired, the bullet will follow the exact same trajectory and hit the exact same spot downrange every time, hence perfect consistancy and therefore perfect accuracy. Of course, we live in the real world where there are so many variables in play, many of which we can't control, that perfect consistancy and therefore accuracy is impossible. Bedding is one of the variables that we can, to some extent, control. By providing solid and consistant bedding, the forces applied on the barrel and/or action are kept the same, shot to shot. If the forces applied to the barreled action are always the same, then the net effect on accuracy is zero, eliminating one accuracy robbing variable from the equation. That is one reason that free-floating barrels is popular. By free-floating the barrel the stock can have little effect on the harmonics and forces applied to the barrel on firing. Combine free-floating with a quality barrel that is consistant throughout it's length and you have the potential for excellent accuracy.

However, you've probably noticed, if you've looked, that there are a number of factory rifles that are not free-floated from the factory (the M700 is a great example). Most of these guns are pressure bedded, or in other words, have the stock touching the barrel at one or more points along its length. Pressure bedding has the advantage of damping the harmonics of the barrel to some extent, which can make a lower quality barrel shoot pretty well with a wider variety of ammo, but the drawback is that if the pressure applied by the stock ever changes, be it due to the barrel heating up, the stock getting hotter/colder and flexing, or warping due to moisture, etc..., it can cause the groups to open up, or the point-of-impact (POI) to shift unexpectedly.

Many factory rifles, and almost all custom/benchrest guns, are free-floated and use some type of bedding system to improve accuracy. Common types are glass bedding (non-permanently mating the action to th stock with a fiberglass/epoxy mixture to fill all the void space between the two), pillar bedding (where there are one or more steel or aluminum "pillars" embedded into the stock through which the action screws pass and the action is tightened down onto), or both. Some dedicated benchrest/varmint rifles are actually permanently glued into specially designed stocks (that have openings to allow the trigger and action to be adjusted/serviced) with epoxy.

So very simply, bedding makes a big difference, and a good bedding job can make a mediocre shooting rifle a lot more accurate. But as the saying goes, you can't polish a turd. If the barrel is poor/shot out, the action is as crooked as a politician, or the ammo sucks, the best bedding job in the world might not make a significant difference.

Mike