My 38 Special Mod 10 4 inch chronographed Remington’s 158 gr rnl R38S5 (year’s ago 5138) load at approx 765 fps. I recall the old constabulary in my New England hometown refer to this load as the Police “Street Load” (or “Police Service”). At least with this revolver 3.5 grs Bullseye and 358311 158 gr rnl duplicated the velocity of Remington’s load for all practical purposes. I’ve seen others use up to 3.8 to 4.0 when duplicating the std 158 gr rnl velocities in their revolvers. I’ve also seen chronograph data of Winchester 158 gr rnl loadings which range from 725 to 770 FPS in different 4 inch revolvers.
But there are many variables as we know it when chronographing loads - temperature, altitude, humidity, cylinder gap, etc. this chronographing business gets complex.
I’ll just say that 3.5 grs Bullseye duplicates the Remington factory load in my revolver.