From Internet Movie Firearms Database - Guns in Movies, TV and Video Games
The following weapons were used in the anime series Vividred Operation:
Heckler & Koch USP
Rei Kuroki uses Heckler & Koch USP several times.
Heckler & Koch MP5A3
Heckler & Koch MP5A3 is the another firearm of Rei Kuroki.
Browning M2 Heavy Barrel
Browning M2 Heavy Barrels were mounted on the tanks.