I suspect that a light 9mm load with 100 grain bullets will still have more kick that 380 loaded with 100 grain bullets in similar size/weight pistols since the 9mm still uses more powder.
A little but not as much as you might think.
I might have to just try that since I reload 100 Grain bullets in my 380 loads anyway.
Slow and fast with 90/95/100s in 9mm can both be fun!
Most of the listed data for 9mm with 100s is pushing them fast to get nice light loads you have to go below the start charge of most data.
This load ran fine in my 1911 with a stock spring, but I couldn't go any lighter and get good function.
I kept backing down until I lost function then added .1gr more than what would function for wiggle room.
5" 9mm 1911, S+B SP, mixed range brass, charges as thrown after setting measure.
(really soft shooting in a all metal 1911)
String: 7
Date: 7/1/2017
Time: 9:24:22 AM
Grains: 100
Hi Vel: 1031
Low Vel: 1005
Ave Vel: 1017.6
Ext Spread: 26
Std Dev: 11.2
4.2HP38 Xtreme 100 1.043 ( Function fine Sig P226 and 1911 )
Velocity Power Factor Ft/Lbs
1016 101.6 229.187
1031 103.1 234.96
1027 102.7 234.176
1009 100.9 226.039
1005 100.5 224.251
some Hodgdon data for .380 with 100s
100 GR. HDY FMJ Hodgdon CFE Pistol .355" .980" 3.3 794 12,400 PSI 4 955 19,800 PSI
100 GR. HDY FMJ Winchester AutoComp .355" .980" 3.8 822 11,000 CUP 4.2 934 14,400 CUP
100 GR. HDY FMJ Hodgdon Universal .355" .980" 3 721 11,400 CUP 3.4 889 16,100 CUP
100 GR. HDY FMJ Winchester 231 .355" .980" 2.9 819 13,800 CUP 3.1 843 15,400 CUP
100 GR. HDY FMJ Hodgdon HP-38 .355" .980" 2.9 819 13,800 CUP 3.1 843 15,400 CUP
100 GR. HDY FMJ Hodgdon Titegroup .355" .980" 2.6 793 11,400 CUP 3 912 14,700 CUP
100 GR. HDY FMJ Winchester 572 .355" .980" 3.4 816 15,300 PSI 4.1 965 19,800 PSI
100 GR. HDY FMJ Winchester 244 .355" .980" 3 851 16,600 PSI 3.5 994 19,500 PSI
so if we call .380 with a 100gr bullet 900fps they are a little hotter than .380 but not by much.
If you put in a lighter recoil spring you could probably back down all the way to .380 levels.