CAUTION: The following post includes loading data beyond currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The High Road, nor the staff of THR assume any liability for any damage or injury resulting from use of this information.
For the advanced and adventuresome handloader that wants the most power he can squeeze out of his 380, and is not a SAAMI fundamentalist, here is what I would tell him what I do.
SAAMI max average pressure for the 380 is 21,500 psi.
Buffalobore is selling +P 380, that I am going to infer from their advertised velocities is 26 kpsi, but scaling off cartridges registered +P it would be less than 24 kpsi.
The proof pressure for 380 is between 28 kpsi and 30 kpsi.
For simplicity in explaining my 380 experimental data I would put 380 pistols in 3 groups.
1) Heavy slide, good case support, good firing pin to firing pin hole fit, and thick chamber walls.
Can shoot 200% extra powder
2) Medium slide, medium case support, ok firing pin fit, and ok chamber walls.
Can shoot 100% extra powder.
3) Fly weight slide, poor case support, sloppy firing pin, paper thin chamber walls.
Can shoot 0% extra powder.
The two most interesting pistols, the Kel-Tec P3AT 380 and the Ruger LCP have the lightest slides, .290" and .270" chamber support respectively, and paper thin chambers.
These are in the 3rd category.
I carry these pistols with loads at the threshold of case bulges starting. No safety margin away from that threshold. That is 26 kpsi in Quickload estimates that agree with the chronograph. They have 2.7" and 2.75" barrels respectively.
So for the hottest loads in the worst pistols for 95 gr FMJ I would load ~5.6 gr Power Pistol, .984" OAL, 26 kpsi, and 1050 fps from a 2.75" barrel.
I would work my way up to that load by catching the brass with a towel wrapped around the pistol. I would examine each case before firing the next. My standard for no safety margin would be to work up the the threshold of case bulge, and then load a bunch more ammo just under that threshold. I would like to have 5 rounds fired at a load and then none of them show more than a couple thousands of bulge, as measured per his picture.