Special44,
Since responding to your thread,I've looked up old records and can now give a more definate answer than before although the gist of it doesn't change.
The reduced load that I mentioned gave an average velocity of 1017 FPS from a 6 1/2" model 29(kind of spooky as the old lyman manual showed 1015).A 3" model 629 gave an average of 834 FPS......183 difference.
A test that Lee Jurras did some years ago is impressive for the reason that he put a 10 inch Douglas premium barrel on a Ruger Super Blackhawk,chronographed three loads and proceeded to test removing one inch at a time.To make certain of consistancy,he used the very same chamber for each shot.There was no change other than the one inch reduction of the barrel between strings of ammo chronographed.He went from ten inches down to two inches.About 400 shots were fired in all.
The medium velocity load will be more relevant to your 44 special question and so I'll limit comments to that.The Remington medium velocity load gave the following results.The ten inch gave 1077 and as each inch was removed they gave 1085,1057,990,1000,973,923,873,655.As you can see,the six inch giving 1000 while the three inch shows 873 makes a difference of 127.Not bad.But as always,it depends on the guns involved.
I've seen revolvers with the same barrel length show well over 100 FPS difference with the same load.