I used the Sierra 125gr JHP ( part# 8125) data when I developed my load with 124 gr RMR-MPR and later RMR Nuke.
I don\’t have the actual sierra bullet but if you go the Blue Collar reloading website , it listed the 125 gr Sierra JHP OAL is .554\”, the RMR Nuke OAL is .545\” so they are similar.
Lyman book suggested COL 1.075\” , HP-38 start load at 3.9 gr and Max load 4.4 gr.
I loaded RMR nuke COL 1.040\” which scary short for my CZ, I start my load at 3.6 gr of HP-38 and worked up to 4.0 gr. It shoot reliable without any sign of high pressure.
I don\’t have chrono data for these load with HP-38 but I do have some data with N320 powder, hope it help.
124 gr RMR-Nuke- Win case , CCI primer
COl 1.040\”, Vihta n320
3.4 gr average 927 FPS
3.7 gr average 983 FPS
3.8 gr average 1012 FPS