The VAST MAJORITY of calibers are tested with a 24" barrel because that is what the SAAMI standard is. The are all built to the exact same specifications and mostly bought from the same companies. Certain calibers like the new 7BackCountry is approved for SAAMI testing out of a 20" barrel so that is what the online or box data shows. It isn't about manufacturer preference it is about industry standard. Any company that posts velocities out of a different length barrel generally will note that.Good idea, but I would suggest 22". Better yet, as others have suggested, provide that data for several barrel lengths - especially those super magnum over-bore loads with a lot of "wallop" (LOL).
The change in bullet velocity per inch of barrel length variation is materially lower at longer barrel lengths vs. shorter. So, if I had to pick ONE barrel length it would probably be 22" because most people who shoot factory ammo for big game hunting will have a 20-24" barrel. Manufacturers would prefer to use 24" to show higher velocities.
However, the more I think about this, given the velocity differential between different barrels (regardless of barrel length) I would tend to say that anyone who needs or wants that much velocity precision should just buy one of those new Garmin Xero's. I have one (I also have an older traditional Chronograph). The XERO's are so easy to use and so compact, I'd say that is the way to go because if you think you need better velocity precision than what is currently being supplied by OEM, then the hunter probably owes it to themselves to get the number RIGHT from their own rifle. I wonder what Formidilosus thinks about my comment.
Either way you shake it to get a proper dope the ammo must be shot out of the individual firearms.
And on a separate subject. Why the hell would companies post information out of a specific manufacturer like TIKKA? The majority of rifle are not TIKKAs? Where do some of you guys come up with stuff?