When introduced the Parker-Hale 'Volunteer' had what they described as being "rifled after the Rigby pattern with eight narrow lands of shallow depth making one turn in 20 inches." That appears to be what you have pictured. These will likely have a serial number less than 1500. Later they replaced this with a version with Henry Rifling, and the serial numbers insofar as I have seen start a new series and are prefixed with 'H'. Henry rifling is polygonal in form and of seven sides with angular projections at the angles - Parker-Hale used 1 in 20 pitch. (Search the web for Martini-Henry rifling and you should find plenty of images).
I have no experience of the 'Rigby' rifled version, but did have a 33" barrel Henry rifled Volunteer (they also made 36" barrel length with Henry rifling). I used the Lyman 45-70 Govt. pure lead bullet (457125) with 80 - 90 grains of TPPH and a plain card wad. TPPH is/was similar to Swiss No. 4 (1.5Fg). The bullet required more sizing then some recommended, but the deep grooves helped I think - irrespective of that I set an MLAGB National Record at 600 yards with it that remained in place for several years, before being beaten by a point.
David