One of the other problems that rifle barrel makers deal with is stresses. They can do funny things to a long hole like that if you cut the steel later. More often than not, these holes are not perfectly concentric or even have some curvature to them for that matter...even in a rifle barrel. The key is that the barrel not change shape after the fact or suffer from lots of stresses or the curvature will change as it warms up. Rifle barrels are usually stress relieved before deep hole drilling if I recall correctly. A barrel maker could handle both for you.