Well, its called an EFR, Extended Front Rail, and it should be done by a qualified smith.
Or if you have a McMillan stock, they have a program written to inlet it.
I know that Louisiana Precision Rifles does them (wmroscoe).
I will tell you that they have a 1.200 ID and your barrel needs to be small enough to fit through it, as they cannot open them up too far. Some people will advise you not to have them opened up at all, as it will compromise the structural integrity by removing some of the hard coat.