Isn't any barrel with variable thickness (such as the MR762 barrel) cut down from thicker material when it's produced in the first place?
It depends on the rifling method used. No matter which method is used though, they all start with the same steps. A length of bar stock is sawn off at a certain length, then it's chucked in a lathe and the ends squared up, followed by center drilling it. Next the bar stock is placed in a gun drilling machine, where they drill a hole several thou' smaller than the intended size through the entire length of bar stock. Then the drilled hole is reamed and honed to the final intended dimension, leaving a perfect mirror smooth finish inside, this process can be done either in a lathe or in most gun drilling machines as well. After all of these steps it's ready for the most important step of all, the actual rifling process. Depending on whether it will be button rifled, hammer forged, or cut rifled, it will be placed in a machine used for that sole purpose, and within no more than a couple of minutes (in the case of button rifling or hammer forging) or a couple of hours (in the case of cut rifling) the bar stock will have been rifled. All that is left to do before this rifled bar stock can be sold to the customer is a final stress-relieve, and in some cases contouring in a lathe. It will also have undergone stress-relieving once or twice earlier throughout the whole process, at least once after receiving it from the steel mill, when it's still raw bar stock, and depending on the barrel maker, once after drilling as well. So yes, unless it went through contouring as the final step, or it was rifled by hammer forging (which also profiles the barrel while it's being rifled) then you will have ended up with a cylindrical barrel blank, which still has to be turned down on a lathe to the final dimensions as used in a rifle. Sorry for the lengthy description, but I hope that it at least clears up part of your question.
