This is the same silliness that folks spread around about short barreled magnum revolvers.
Magnum pistol rounds also suffer considerably in short barrels. I have 3", 4" and 6" .44 magnums (and a 20" 1894 carbine, but that's not material to this discussion), and there's a big difference. I don't have my load data records in front of me right now to give exact readings and averages with the various loads, but the sledgehammer 240 gr. loads out of the 3" 629-2 were actually lower energy than 180 gr. 10mm loads from a 5" semi auto. A .44 mag loses about 300 FPS going from 6" to 3". That's a huge drop when you were only running 1,300-1,400 out of the 6" gun. Also have 2", 4", and 6" .357 magnums, as well as carbines, have had 3 and 8 inchers in the past; .357 mag suffers even more. If you don't like my figures, go check out BBTI.
No, .357 out of a 3" barrel doesn't drop down to .38 spl levels as some have suggested at other times. But it's also running at twice the chamber pressure. That's not the case when comparing 55-60KSI rifle rounds. I'm not and never have suggested the .22-250 becomes a Hornet in a short barrel, but I am saying with absolute certainty that it suffers much greater velocity loss in a shorter barrel than a more efficient round like the .223, and there is a point that the extra powder turns almost entirely into muzzle blast instead of velocity. At 18", .223 gets within 100 FPS of the .22-250. At 14", your shot to shot variation is likely to be more than the average difference between the rounds.
.243 will lose 250-300 FPS going from a 22" gun to a 16" gun. .308, OTOH, loses about 150 FPS average with the same 6" chop. That doesn't mean a 16" .243 is ineffective or useless, but it does mean that it suffers quite a bit more, and that's the point I've been trying to get across.
This stuff is not speculative. It's material fact, and has been tested by far more people than myself. I also personally own weapons in many of these chamberings, and would wager that I've done more with barrels of different lengths in them than just about anyone on here, save for Clark. I have a .243 that we cut, have built and shortened others. I have done 6.5 CM builds & chops from 12" to 26". .223 from 7" to 24". .22-250 from 20 to 26". .308 from 8.5 to 28". .30-06 from 18-27". .300 win from 18" to 26".
I also seriously doubt that is true, but I'm not cutting my barrel to see. I can say without a doubt that my 20" 6 Creed is as fast as the guys running 27 and 28" BRAs and Dashers.
You're talking about what is really the minimum practical length for .243/6 creed vs. quite long barrels for the stubby benchrest rounds that offer rather little velocity advantage over a 20" or 22". And none of those are the lengths we were discussing as handicapping overbore rounds. 6BR and 6 Dasher behave more like .308 win, gaining or losing only 30 ish FPS per inch in the 18-26" range. Above 26", they gain very little, down to single digit by the time you're approaching 30".
As well, the 6 creed from a 24" gun has a 300 FPS advantage over the 6 BR in the same 24" tube. So yeah, losing 150-200 FPS going down to 20" with the Creed vs. gaining only 20-25 FPS per inch from 24" to 27" or 28" with the BR or dasher, the math works out just fine. Point remains that the BR or Dasher will suffer
less velocity loss going
down to 20" because they are more efficient cartridges. I feel that you are being willfully obtuse about this and creating deliberately flawed arguments to support an untenable assertion. You're also being rather hostile in your replies; I will not continue this discourse if you cannot be dispassionate and objective. We're discussing cartridge ballistics here, not politics or morals. We all like what we like, but being disingenuous due to your preferences doesn't bolster an argument.
Now, one more time, the powder charge to bore area ratio has a direct correlation to the velocity gain/loss with different barrel lengths. The effects are more profound as you move further from the ideal barrel length in either direction, and the lower that ratio, the shorter that "ideal" barrel length (best balance between portability & performance) can be. And that brings us back to the thread topic of why the .243 isn't very poplular in ARs, which a LFAR gets rather unweildy for most of its purposes with barrels over 20". 18" barrels on a LFAR give an OAL about the same as a 22"-24" bolt gun, which is why it's such a popular length. The more dramatic velocity loss of the .243 vs. .308 in that shorter barrel is one of the reasons it's not a popular choice. Ammo price and availability are a larger factor, but the contribution of external ballistics should not be ignored.