I think there are different definitions of "great" at work here. I am primarily thinking of originals, others are thinking of contemporary working guns, etc.
Then there is "that's a great rifle" meaning really fine, versus, "that's a great rifle meaning "I'll remember that one; I'll think about it, I'll go over it and enjoy this and that about it. I've seen plenty of rifles that are generally of that style, some really fine ones, but this one is stands out."
I'll blaspheme here and say there are some Becks and Schroyers and Dickerts that are not great in my eye, although I esteem them because of their place in history and who made them.
To get at "soul", why are some songs great and some not? They are unforgettable, but why? They are coherent, but also have a "catch", something special that sticks. It might be the lyrics you remember, it might be the tune of the chorus, it might be the bridge, the bass line or a guitar solo. A great song may have lots of memorable and outstanding features, but the greatest will have a number of memorable features. The parts have to work together. Same may be true for great rifles.