To make progressive depth rifleing as was done originally required the rifleing cutter to be made different depths during the cut. The only way I can see that done is to have an additional rod that holds the cutter depth screw from turning with the cutter. That would move the cutter wedge in and out along the length of the barrel. How much could be determined by the thread pitch and steepness of the wedge.
I am pretty sure that Parker Hale used hammer forged barrels. The mandrill had progressive depth. This is much easier.
The ironic thing is the most repro guns use extremely shallow rifling, about 0.004" and less., if I remember right from a video I saw. The bullet jumps the rifling very easily. That shallow rifling does not work with period correct projectiles and loadings. So, most reproductions are rifled incorrectly for decent accuracy with traditional hollow base bullets. Ironically, period testing from the 19th century showed that deeper rifling, about 0.015", but constant depth gave excellent accuracy. The progressive rifling was only slightly better. So your reproduction is intentionally rifled wrong and the maker does not care.
Bobby Hoyt is a master at making accurate barrels and liners for rifled muskets.
