Of course.
I'm not someone who knows (or cares about) every stinking detail of the Kennedy assassination. However, once it was established that the serial number of the murder weapon matches the serial number of the weapon shipped by Klien's, it's apparent that all the quibbling about the barrel length is a complete load of nonsense.
I'm going to invoke Occam's razor: Which is the more believable scenario?
1. There were two weapons of the same make and model, but different barrel lengths, with the same serial number, one of which was shipped to Oswald, and the other of which was used (by somebody else?) in Oswald's place of employment to shoot JFK.
2. Klein's substituted the 40" barrel rifle of the same make and model that they had for the 36" barrel rifle that Oswald had ordered, and failed to note (either through lack of care or because they were hoping that the customer didn't care or wouldn't notice).
I don't think anybody who has any experience with mail order merchants would find scenario number two the least bit hard to believe, whereas scenario number one requires not only that the manufacturer had some sort of bizarre serial number policy or by some strange error made the mistake of assigning the same serial number to two different weapons, and that by a coincidence of extremely low probability, one of these two weapons (out of how many thousand?) was shipped to Oswald and the other was used to assassinate Kennedy, in Oswald's workplace.
I know which scenario I believe and I'm pretty sure it's the same scenario that any reasonable person would believe.