A Handloader magazine recently ran an article describing Phil Sharpes test of a Springfield rifle with a extra long (36"??) barrel.
He cut it down, one inch at a time, and recorded velocities.
Velocities really didn't seem to drop much until he got below 17 or 18". In some cases they went UP between 25" to 22"
I'd say it makes no real difference. Even in most magnum catridges, a 2" difference in barrel length may not give you much extra loss/gain.
As Rembo said, it coudl be the same variation betweeen 2 identical barrels, same lenght.