First, let us recall our discussions about various types of stocks from a couple of months ago: wooden stocks, injection molded stocks etc. Wooden stocks have a problem of warping, due to humidity and temperature variations. Some other stock types are not vulnerable to humidity, but will also warp with temperature variations. As a result, they could cause the accuracy of the weapon to be affected. One way we saw to eliminate these issues, is to lay a bedding of a temperature-insensitive epoxy bedding material. The epoxy material ensures that the barrel contacts the entire stock at all points and this contact does not change with temperature variations. Therefore there is a constant pressure dynamic between the barrel and the stock throughout the length of the barrel, which also improves accuracy. Another method, which we mentioned in the post about bedding materials (but did not elaborate upon then) is to only attach the receiver mechanism and the parts behind it to the stock, and leave the barrel floating in the air, without touching the stock anywhere. This is what is meant by "free floating".
When a weapon is fired, there are vibrations induced in the steel barrel as the bullet travels down it. If the barrel is touching the stock (which may be made of wood, metal, plastic etc.) in a non-uniform manner, the barrel vibrations may be dampened differently from shot to shot, which changes the point where the bullet strikes. Depending on where the hand holds the stock and how loosely or tightly the hand grips the stock may also affect the vibration dampening characteristics and thereby the accuracy of the weapon. With a free-floated barrel, there is no contact between the barrel and anything else. Therefore, there is nothing that interferes with the natural harmonics of the steel barrel and the pressure dynamic is constant throughout the length of the barrel.
Another reason to free-float is because barrels tend to heat up when they are fired, which causes the barrel to expand. If the contact points of the barrel and the stock are not perfect (e.g. if no proper bedding is done), the barrel will bend slightly across the high contact points and therefore mess with the accuracy of the weapon.
A quick way to test if a barrel is free-floated or not, is to slide a piece of ordinary paper between the barrel and the stock. If the barrel is properly free-floated, the user will be able to slide the paper all the way down the length of the barrel. If the paper cannot be slid all the way, that means that some portion of the stock is contacting the barrel.
The following video shows how to check if the barrel is really free-floating as advertised and how to fix problems:
Of course, free floating a barrel does not immediately mean that accuracy is improved. It usually works best with heavier, large caliber barrels. With lighter barrels, they tend to flex more when a shot is fired and since the barrel isn't touching anything else to support it, the angle of flexing may be more, which could cause the shot to go to a different point from where the user is aiming. Therefore, thinner barrels are more improved by glass bedding, rather than free floating the barrel. However, glass bedding is a more expensive process. In some cases, users glass bed the first few inches of barrel after the receiver and then leave the rest to free-float.
Free floated barrels are generally used for highly accurate weapons, such as match-grade rifles and weapons used by snipers.