Fred,
A great deal depends on the gun itself and the types of loads used. Blanket statements like "Each inch costs XX feet per second" just don't cut it. As far as the gun goes, I've found cylinder gap to be the single most influential factor in revolver velocity. I have tested several 2 to 2.75" snubbies with tight gaps that were faster over the chronograph than 4" guns with big gaps using ammo from the same box. Whenever I seriously go wheelgun shopping I always carry a feeler gauge. My personal limit is .006" on a rare model, .004" on more common guns.
As far as ammo goes, it's been my experience that loads using heavier bullets will generally lose less velocity from a snubbie than lightweights. The type of powder being used also has a great deal of impact. For example, my long-time carry round in .357 Magnum snubbies has been Remington's 125-grain "Medium Velocity" load. It uses faster powders than the full-power offerings. Chronographed through several
snubbies the Medium Velocity load nearly matches and on one occasion exceeded full-power 125's in the same guns. The full-power loads really don't start to strut their stuff until fired from at least a 4" gun.
For your .38 and .44 Special loads I'd try powders like Unique or Universal for starters. Using lead bullets will also help keep your velocities as high as possible.