There were two or three articles in Gun Digest about the late British muzzleloaders, the Volunteer Rifles and the "smallbores" by Whitworth and competitors. I'll see if I can dig them up.
Got 'em.
The Whitworth Rifle, a Great Milestone in Rifle History. Gun Digest 1971
Also
British Volunteer Rifles 1850-1870, Gun Digest 1972. (These were nice .577s, Italian repro merchandising notwithstanding.).
British Small Bore Rifles Part One, the .451 Muzzleloaders. (Didn't make it to Part two.)
All by DeWitt Bailey II.