Yes, plain economics does not allow gun companies to offer several barrel lengths in most cases.
Now, if it were true that
All cartridges lost 20fps per inch reduction, then
All of my rifles would have short barrels, period. Problem is, it is not true.
On the other hand, if
ALL cartridges lost 50fps per inch reduction, then
ALL my rifles except maybe my 45-70 would have long barrels. Problem is, it is not true.
These are extremes, but I have seen very little loss, I mean less than 20fps (almost none sometimes) with a couple of rifles, a 45-70 &
a very nice looking 444 come to mind. But on the other hand, a friend of mine bought a 7mmMag Vanguard years ago with that stupid
20" tube(stupid for a 7mag) & it was a joke, not counting the muzzle blast. I have checked enough 25-06's to know the story there as well, esp. with the slowest powders & the heavier bullets. I agree about the slow powders vs fast vs barrel length talk to a degree, but when you combine the heavier bullets & slow powders in some of the so-called "overbore" rounds, it matters at that point as I found out with the 125 Wildcat bullet in 25-06AI. If you think you can do well with the 22-250AI or 6X284 with an 18" tube, then have fun.
Some of the companies like Rem who years ago got lazy & made a broad brush statement that 20fps covers it with rifle barrels have not helped much. In the std. rounds like the 308 this is not far off, in fact I have thought of cutting my 308 tube from 26" to 20" because
it doesn't lose much, as seen here:
http://www.6mmbr.com/308Win.htmlSo, what does this all mean? Research the cartridge you are interested in & the info is out there, but the so called xfps per inch no
matter what stuff, well forget about it.