As stated, most (all?) of the fixed sight S&W 38 Specials were/are regulated for 158 grain factory loads.
An easy approach for handloaders is to check the
factory load ballistics here.
Then work up a handload that delivers comparable velocity.
A 158/160 LRN over 4.0 grains of WW 231 is a popular factory duplication load. In my 4" guns it gives about 775 FPS.
Most of mine seem to shoot the round nose bullets a bit more accurately than the SWCs.