The problem with the .380acp is not that it needs to use lighter bullets, but that it needs to use longer heavier bullets. At 90grs it has a sectional density lower than that of a .22lr or .25acp resulting in very poor penetration with JHPs. Sectional density in lay terms is basically how much momentum a bullet possesses relative to its diameter. A 9mm 135gr bullet has more sectional density than a .40 135gr which focuses more momentum on a smaller focal point resulting in greater penetration with JHPs.
Sectional Density is what the .380acp needs.
"Many small, easily concealed semi-automatic pistols which are recommended for law enforcement backup or concealed carry use fire .380 ACP or smaller bullets. While these small caliber handgun bullets can produce fatal wounds,they are less likely to produce the rapid incapacitation necessary in law enforcement or self-defense situations.
Handguns chambered in .380 ACP are small, compact, and generally easy to carry. Unfortunately, testing has shown that they offer inadequate performance for self-defense and for law enforcement use whether on duty as a back-up weapon or for off duty carry. The terminal performance of .380 ACP JHP's is often erratic, with inadequate penetration and inconsistent expansion being common problems, while .380 ACP FMJ's offer adequate penetration, but no expansion. All of the .380 ACP JHP loads we have tested, including CorBon, Hornady, Federal, Remington, Speer, and Winchester exhibited inconsistent, unacceptable terminal performance for law enforcement back-up and off duty self-defense use due to inadequate penetration or inadequate expansion. Stick with FMJ for .380 ACP or better yet, don't use it at all. The use of .380 ACP and smaller caliber weapons is really not acceptable for law enforcement use and most savvy agencies prohibit them." -Dr. Gary Roberts at the National Defense Industrial Association
Winchester Ranger Talons:
FBI Test Protocol: (12" of penetration is passing)
Bare Gelatin = covered with T-shirt, Shot at 10ft
Denim = 4 layers shot at 10ft
Heavy Clothing = shot at 10ft
Steel = 2 pieces of 20 gauge, shot at 10ft
Wallboard = 2 pieces of 1/2" gypsum board, shot at 10ft
Plywood = 1 piece of 3/4" AA fir plywood, shot at 10ft
Automobile Glass = 1 piece of 1/4" laminated safety glass set at a 45 degree angle with an offset of 15 degrees, shot at 10ft
.380 95gr(1000fps): (Penetration/Expansion)
Bare Gel: 7.65”/.65”
Through Denim: 7.95”/.64”
Through Heavy Cloth: 7.85”/.64”
Through Wallboard: 15”/.36” JHP plugged with material, load behaves as FMJ.
Through Plywood: 15.5”/.36” JHP plugged with material, load behaves as FMJ.
Through Steel: 9.3”/.36”
Through Auto Glass: 4.5”/NA
This is typical performance of the .380acp with most modern loads. Increasing velocity in the case of Double Tap ammo only improves expansion of the GD bullet, it does not improve penetration as was observed by CCI Speer when they designed the Gold Dot. A truly improved load for self defense would be a .380acp JHP with a bullet weight of around 100gr that expands reliably. The only load I've seen accurate testing for that borderlines passing in bare gel is the Hornady 90gr XTP which I carry as my BUG on duty. It does this by limiting bullet expansion to around .45" so as to not have too much of a parachute effect and reduce penetration.