 In 1906 and 1907, the U.S. Army held trials for a large-caliber semi-automatic pistol, which constituted the fourth and final evaluation of the Luger or Parabellum pistol design by U.S. authorities.At least two, and possibly three Parabellum Model 1902/1906 pattern pistols in enlarged form and chambered in .45 Luger caliber were brought to the U.S. by Georg Luger for the 1907 trials, each chambered in .45 ACP caliber.Prior to his arrival, the U.S. Frankford Arsenal had provided Luger with 5,000 rounds of .45 ammunition for experimentation and to serve as a guide for chambering measurements.