In fairness to Mann, in the books Killdeer is noted as being an exceptionally long rifle, even by the standards of a later day, to the point that Hawkeye is actually named by his enemies after the length of his rifle. Thus, I don't think that a shorter barrel would have worked from a storytelling point of view (though considering what Mann did the the rest of the story, that seems a mere quibble....
Going from memory, Killdeer is never actually described in
Last of the Mohicans, but from bits in
The Pioneers and
The Deerslayer indicate that it was around six feet long overall, had a brass box and and a bore around 30 to the pound (in the early 1790s, after about fifty years of active service and several freshings-out), and had a few bits of silver inlayed but was overall a fairly plain piece. When Judith Hutter gave it to Natty in 1745, it had been in her father's possession long enough to have already won a reputation as an exceptionally well-made and accurate piece, which suggests it wasn't built much later than 1740.
Cooper was obviously thinking of a Golden Age rifle, which makes sense given that most of the above comes from the first book he wrote set in 1793, in which Natty is just an old hunter meant to represent the old frontier giving way to the forces of civilization - Natty kind of stole the show, so to speak, and ended up a recurring character in books set in earlier periods. Killdeer kind of naturally tagged along, and became a character in its own right, out of place from its original description. However, one important part of Killdeer's description that tends to get overlooked is that it was never intended to by a typical Kentucky rifle, just as Natty is not a typical frontiersman (as a matter of fact, most of Cooper's frontiersmen are not very sympathetic characters at all) - Killdeer is long, lean, rather plain if not homely, but of exceptional quality where it counts, just as Natty himself is rather homely and often pretty rough in manner, but also a shining example of untutored virtue free from the artifices of civilization. A unique weapon for a unique man.
I kind of want to make an interpretation of Killdeer someday: not the movie version, which doesn't appeal to me, but a gun that is historically plausible but fits the spirit of Cooper's vision if not the literal description - long heavy barrel (possibly off a fine German bench-rest rifle, which would explain the longer length and superior workmanship, though if one of those might come light enough to be used as an ordinary rifle by a tall, strong man like Natty I don't know), high-end lock with springs "like a wolf trap," but stocked here in America in sugar maple with plain, unfussy lines and just a bit of carving in a provincial manner.
