https://www.everydaymarksman.co/equipment/rifling-twist-rate/
A newer technology, Electrolytic Cationic Machining, uses a wet-etching
method that uses reverse-electroplating to remove from inside the barrel
rather than add to it. These machines utilize electrodes shaped as
plastic cylinders with reverse-imaged metal strips encircling them. To
create the desired twist rate, the cylinder is pushed through the barrel
while the barrel is immersed in chemicals like sodium nitrate and
methodically rotated. Although this is an expensive method of rifling,
it results in precise rifling.
