Interesting from what I have read 2" of barrel equals about 150fps of velocity so the difference between a M1A Standard and Scout is about 300fps.
I don't doubt that someone found this to be true. But comparing velocity between 2 different guns tells you nothing. I've seen 130 fps difference between guns with equal barrel lengths shooting the same ammo. Some barrels just shoot faster. If you happen to compare a "slow" 20" barrel to another gun with a "fast" 24" barrel 300 fps is possible.
But on the other hand if the 20" barrel happens to be on the "fast" side, it could very well shoot at the same speed or faster than a "slow" 24" barrel. I've seen a 2" shorter barrel shoot faster with the same ammo several times.
Between 20-24" I've found around 1/2%-1% velocity per inch to be pretty accurate. If you are getting 3000 fps from a 24" barrel, then you'll lose about 15- 30 fps for each inch you cut it shorter. If you are looking at barrels longer than 24" the gains are even smaller. When you go shorter than 20" the loses are greater , but no where near what is often repeated on the internet.
And this only applies when cutting a barrel shorter that has a known velocity. When comparing 2 different guns, there is no guarantee a 24" barrel will actually shoot faster than another gun with a 22" barrel.