In Shooting Times of 16th August 2007 (page 24) there was an article written (I think) by Pete Moore. He made practical tests with a .308 and a .223 by progressively shortening the barrels and measuring the MV over the chrono. He compared it with the computer generated predictions of 'Quick Load'. For all practical purposes the results were very similar.
Practical test for .223 gave 24'' 3,402 fps mv, whilst 20'' gave 3,253 fps mv and 18'' gave 3,185 fps mv. Using a 52 grain bullet.
The .308 gave 24'' 2,905 fps mv, whilst the 20'' gave 2,851 fps mv and 18'' gave 2,748 fps mv. Using a 150 grain bullet.
Reducing from 24'' to 18'' the computer prediction was a loss of 194 fps whilst the practical test showed a loss of 217 fps for the .223. Difference 23 fps.
The .308 predicted loss was 189 fps whilst the actual was 157 fps. Difference 32 fps.
It was after reading the article that I had my Sako 75 .243 shortened to 20 inches and ordered my Sako 85 .30-06 with a 20'' barrel.
One cautionary note with the .243 is whether it will generate 1,700 ft lbs muzzle energy with the shorter barrel. Some factory loads, particularly 100 grain could be maginal as they do not meet the claimed mv when tested over the chronograph. I home load using a Speer SPBT 85 grains at approx 3,100 fps mv, so no issue for me on that score. I hope you find this useful.