I have a mod 19, and reload both 38 and 357. Here's the question...

In any particular reloading guide, For 38, with a particular bullet weight, there is a linear relationship between the grains of powder and the velocity. The same is true for 357.

The difference seems to be between the two. If I interpolate between the two pages, the relationship is not linear.

In 38, an additional .2 grains of powder may give me an additional 50 fps. In 357, an additional .2 grains of powder may give me an additional 50 fps. The question is that the difference in powder loads between the max 38 and the min 357 is not .2 grains per 50 fps.

Am I safe in assuming that this is due to the difference in the length (ie. case volume) between the 38 spl and 357 (assuming bullet seating is at the same place)?