If anyone has the time could they explain to me how does this modification help accuracy?
If the stock puts pressure on the barrel in front of the action the barrel flex in a different position each time you fire.
Relieving this pressure by sanding out the stock barrel channel and add shims to raise the barrel eliminates this problem.
This video references wood stocks but the same principle works with synthetic stock.
This video will help you understand.
http://www.youtube.com/watch?v=MGEh_GYgZqk