The .380 LCP and the like fit a niche that 9mm can't fill, and that is truly a gun that can fit in any pocket and go any place. While there are smaller 9mm on the market none are as small as the LCP.
I bought mine as a niche gun for when I can't carry some thing bigger and even after buying a compact Walther PPS 9mm my LCP still sees the same amount of carry.
Ruger also introduced a new .380 in a larger frame. I assume that due to the growing numbers of CCW carriers, the .380 offers less recoil for novice shooters in a frame size that is easy to shoot.
If any thing, I would guess that the .380 is growing in popularity or in the least has remained steady. It certainly has not declined.
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