Originally Posted by
Symmetry
Simply put, there is currently not a caliber in the .380 sized platform that is capable of meeting minimal penetration standards while expanding to at least 50% of its caliber size. The .380 cartridge is not capable of being loading with adequate bullet weights and designs, it has limited case capacity, and it has low pressure limits. The 9mm remains the most mechanically efficient caliber to date in these regards.
What would be more effective in the .380 sized platform would be a smaller caliber cartridge capable of using longer bullets, and operates at higher pressures. I've always that that a .30-.32 caliber bullet with a sectional density of .170, a pressure limit of 35k-40k psi, and a velocity around 1000fps would be the ideal cartridge to fit in the .380 platform. It would also allow for a double column magazine, so a small subcompact with at least 10rds in the mag.