Hi lethian,are you talking about a hybrid pipe gun? if so maybe the way to go is not to bore a hole for the barrel but to build the gun around the barrel?, you could build the gun in two pieces and glue the two halves together around the barrel, simply build the two halves of the stock and route a groove or channel in the two halves and glue them around the barrel, you could use a carbon fibre tube or even aluminium,make it a good fit and if you route a square channel the epoxy will fill the gaps and still be remarkably strong , make sure you sand aluminium tube just before gluing to make a good bond and to prevent and corrosion from forming on the surface, make sure that you have a good length of the barrel in the timber for strength, they have been bonding aluminium in the aerospace industry for years , west system epoxy or something similar works well, if you are trying to make an enclosed barrel you must use a router with a guide or better still use a router table , you can get cutters from a web site , internet custom spearguns , it is a good site with lots of excellent advice,let me know how you go , I am very interested ,ANDREW