In H&R 223 caliber I have
1.) a bull barrel serial number "NS******" with a twist rate of 1 in 9
and
2.) a standard barrel serial number "NC******" with a twist rate of 1 in 12
One way to determine the rate of twist is to push a cleaning rod (with bronze brush) from muzzle to chamber. Loosely roll a cleaning patch onto the brass brush as it sits in the chamber. Mark the cleaning rod with a marker at the muzzle in line with the barrel (<==important).
Slowly withdraw the cleaning rod (allowing it to engage the grooves and twist) and watch as the mark rotates one full turn around. Measure the distance from the tip of the mark (which was formerly at the muzzle) to the muzzle. It is a long gun and not a handgun so there won't be any partial inches to consider. For a 223 it will either be 1 in 12 or 1 in 9.