I have the USPc .45, and will soon acquire one in 9mm. The .40 S&W caliber does not interest me in the least.
I have shot all three calibers; in fact, a pal's FFDO .40 USPc is what got me interested in the pistol. IMO, the .40 has noticeably more recoil/flip than either the .45 or 9mm.
Be aware that the 9mm and the .40 are a bit smaller, in OAL and length of the grip, than the .45. Not much, but enough to notice. Width and height are the same on all. I have a small hand, and the 9mm/.40 grip feels better to me. But I have been shooting the .45 since July, over 5K rounds, and I have "adapted" to it.
I like this pistol immensely. My Glocks which have served me faithfully since 1992 are now gathering dust.
FWIW... there is a reason that CDNN and others have the .357 Sig caliber guns "on sale". I do not know what that reason is, but I suspect slow sales has a lot to do with it. The only thing I know for sure is that they are not reducing the price out of the goodness of their hearts...
With good ammunition, the USPc is extremely accurate. And the legendary HK reliability is just that. The only malfunctions (five total) I have experienced were due to too-lightly-loaded ammunition when I was experimenting to find a good, soft practice load.
I have never understood this capacity thing. IMO, the .40 is neither fish nor fowl, although many folks are fond of it. To me, if you want a lot of bullets, carry a 9mm. If you want big bullets, carry a .45. If you want a lot of big bullets, carry an M1A or FAL or G3...
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