when the barrel is free floating the vibration patterns will stay more concistant,if the barrel vibrates the same every shot your groups will be tighter.when the stock forarm touches the barrel,the barrel vibrations will be different ever time you pull the trigger.this intern will change to point of impact from shot to shot ,making group size larger.as gtbuzz said free floating alone may not do much,bedding the reciever,recoil lug and free floating the barrel will in most cases make a very noticable difference in accuracy.the idea is to keep the stock,reciever and barrel all in the same place,no movment every shot.weather conditions also couse stocks to shift changing point of impact.glass bedding will elimanate this movment.