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Barrels can be made different ways . You can take a bar and drill it out. You can take a short bar [~12"] drill it out and rotary forge it ,which stretches it out to ~ 24". The rotary forge method IIRC was developed by Steyr at least 30 years ago and many manufacturers have used it. Rifling can be cut with a broach or more commonly "button rifled" where a carbide button is drawn through the barrel cold forming the rifling. Since free maching grades of steel are not recommended for much cold working that should exclude 416 from rotary forging. The trick is to get a straight hole [ using a single straight flute drill called a 'gun drill' ] that is centered in the barrel. The outside can easily be machined , or ground and flutes can be milled.
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