A .45 automatic was a semi-automatic hand-held projectile weapon used during the mid-to-late 20th century on the planet Earth.
In 2375, Chief O'Brien commented that due to the jack-in-the-box planted in the Bashir 62 holosuite program by Felix, the character Frankie Eyes would need to be killed by a period specific weapon like a .45 automatic and not a phaser. (DS9: "Badda-Bing, Badda-Bang")
".45 automatic" was a common name given to the M1911 pistol.