You don't say what barrel you are putting on it, but I would bore the breach about a half inch deep do about .975, leave the rest at 800 and then turn the barrel to .800 leaving a sholder where the front of the stub meets the barrel. If the barrel is too small, turn it to 800 all the way through the stub and thread on a nut from the breach end, this is how I did the last 25-35 barrel, then make a nut with the OD of about .970 with threads in it for the .800 barrel, screw it all together and you will be all set. Larry