With the light factory loads you will not see a huge gain, perhaps 50-100 fps. While the .44 Mag gains around 175 fps going from a 6" to a 10" barrel (G&A June 1988 pg. 47), it is loaded to much high pressures. A .45LC, which is loaded to around the same pressure as a .44 SPL, gains 103 fps from a 6" to a 10" barrel. For the .38 SPL, it's about 73 fps.
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