Patrick K. would know, but I haven't seen him on here in a while. He posted a link to a great article about the project a long time ago.
FWIW, the rifles used in that experiment were almost certainly chambered in 6PPC, and were probably bull barrels or more correctly, NBRSA- legal taper, which amounts to 1.25" diameter the first five inches, and tapering to .900 over the next 24", or .900" at 29". Although virtually no short range bench rest rifles have 29" barrels, so a shorter barrel can have a larger muzzle diameter as long as it follows this taper. So the taper of the barrel as well as the chambering would affect the harmonics in my opinion. I don't think that the 21.75" ideal length is meant to be universal.
If you're building a .243AI, I'd assume you intend to shoot it at distances longer than 100-200 yards, and if so, my opinion is that it'd be smarter to use a longer barrel to gain all the additional velocity you could from a relatively overbore cartridge. With a 21.75" barrel you'd be leaving a lot of velocity on the table.
If you really are only shooting shorter distances, do yourself a favor and build a 6BR.
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