I suppose your substituting your preconceptions for the recorded test results of the 2,400 yard Springfield test is the difference between us. The British head to head tests between the Enfield & the Whitworth showed even more dramatic disparities than did those with the Springfield. The Enfield had serious problems due to relatively poor quality standards. This was recognized by all concerned at that time. As a result, Enfields consistently did poorly in head to head tests with contemporary rifles. I don't just make things up.
Would you be able to provide those recorded test results? I'd be willing to pay attention if they were provided, albeit I'd prefer as primary a source as possible, but on an immediate viewing it sounds like something sensationalized because it only appears in Scientific American and is described as a "private test" - what the reader gets is the interpretation by Scientific American, not the raw results.
My suspicion about what happened is that someone got one fluke hit at 2,400 yards with the Springfield, none with the Enfield, and decided that that reflected real performance; at 2,400 yards the bullet spends more than six seconds in the air, and is going to hit with only about the force that gravity can give it. (The Enfield would be the same, as the bullets of the weapon are nearly identical; the Whitworth is a much longer bullet so retains more velocity at long range owing to higher sectional density.)
As for the performance of the Enfield, I don't have a problem with the idea that the Enfield started to struggle at ranges of half a mile. Certainly the evidence from British qualification tests in 1860 indicates it was hardly useless at 900 yards (just less efficient than at 500) but at 900 yards the British limited its use to 1st Class shots who could get the most out of the weapon.
Certainly I can point to comparative Confederate tests in which they found the Enfield was the better weapon for marksmanship:
Major Dunlop, responsible for much of the intensive training that was given to the Confederate sharpshooters, wrote of a comparative test undertaken between a number of issue rifles including Springfield and Richmond rifle-muskets and the Pattern 1853.
“The superiority of the Enfield at long ranges, from 600 to 900 yards, was clearly demonstrated both as to force and accuracy of fire. The Enfield proved reliable and effective to a distance of 900 yards, while the other rifles could only be relied on at a distance of 500 yards.”
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I do however think I'd prefer a machine-made (i.e. more standardized) late-model Enfield (such as the machine made versions of the P1860) to a late-model Springfield for marksmanship work, because the maximum setting on the ladder sights on the Enfield went from 900 yards to 1,000 yards to 1,250 yards over time while Springfields such as the 1861 used less accurate flip-up leaf sights for 300 and 500 yards. That by itself makes any kind of deliberate hit at 2,400 yards extremely difficult - there is simply not the setting on the sights for it.