There have been many people who have chronographed shots out of a barrel as it was cut back a little bit at a time. People in the USA should be wary of duplicating these tests because once the rifle barrel is cut shorter than 16" an unlawfully manufactured Class III "short barreled rifle" is the result.

A most comprehensive test was performed by someone who posted his results on Accurate Reloading. The report was followed up by a reader who graphed and discussed the AR data. Here are three of several tables from the discussion:

It is interesting to note in the first chart that for the .223 tested there appears a flat spot in the velocity curves at around 16"-17" barrel length. If you look at the second chart you will see accuracy hit a "sweet spot" when the barrel was approximately 17" in length. This suggests that there may be an ideal barrel length for efficiency. Perhaps there is an inherent ideal for each caliber and/or bullet. The third chart seems to support this as well - note how and where the lines cross. I would much rather shoot a bullet a little slower and get the ultimate accuracy than push it to maximum velocity with accuracy be damned.

The AR data is here: http://www.accuratereloading.com/223sb.html

The full report of the above data with more charts is here: http://www.varmintal.com/alite.htm#BarrelLength