In 1858, the Enfield and Whitworth Committee recommended the manufacture of eight Enfield rifles of a caliber suitable to fire Whitworth's small-bore cylindrical lead projectile so that accuracy tests could be conducted between these two weapons. Two calibers, .451 and .453 in., were tried. The military potential of these first small-bore Enfields was made so apparent that the Whitworth and Enfield Committee proposed that, before the next year, 25 Enfield small bores should be procured with 39 in. and 10 further with 33 in. barrels.
These later small-bore Enfields were mainly of .451 caliber and rifled with five grooves making one turn in 20 in. The smaller bore and thicker barrel made these small-bore Enfields nearly a pound heavier than the service Enfield. In the small bore trials of 1862 the Enfield was found inferior in accuracy of fire to the Whitworth, but only by the slightest margin.
This is a condensed version of information found on pp. 133-134 of C.H. Roads' book "The British Soldier's Firearm, 1850-1864: From Smooth-bore to Small-bore." A number of similar small-bore rifles manufactured by various English gunmakers (Henry, Turner, Rigby, etc.) appeared about this time. It may have been one of these .45 caliber Enfields that Weller was firing in his comparative trials of Civil War longarms for his American Rifleman articles published in 1954 and 1971.