Originally Posted by AJK1
Contrary to popular belief, you should never stick or add anything inside the handle or buttcap. This method actually makes your racquet unstable in it's swing path. Only add weight (lead tape) to the outside of the handle, underneath the grip, and only above the 10cm mark.
Then I guess many pros love unstable rackets and many professional customizers love creating them...
Silicone in the handle is a very frequently used method to add weight, and is done INSIDE the handle...