Morning all.
It's a null question as to whether the rifling is applied from chamber or muzzle side of the barrel. The intention is to still have a consistent rate-of-twist from start to finish (unless it's a barrel with gain-twist rifling), and the rifle blank, prior to throating and crowning, is identical over the entire length. This applies to any barrel, irrespective of manufacture method (hammer-forged, button-rifled or cut rifling).
Also, unless you're creating an artillery piece barrel, which has to be able to compensate for the coriolis effect, the direction of twist has an utterly minimal impact on accuracy - so manufacturers just make sure the direction of twist is the same in all the pieces they produce.
*edit*
In case it isn't clear, to change the direction of rifling (left-hand vs right-hand twist), you'd use a different carbide rifling button, or carbide mandrel, or hook cutter. The direction of cut on these would be different for LH vs RH, but which direction you pull/push/hammer/cut in is still irrelevant.
