The choice of calibre took some time to be settled. As large quantities of Colt .45 ACP cartridges were available; it was decided to adopt the Owen gun for it. Official trials were organised, and the John Lysaght factory made three versions in 9×19mm, .38-200 and .45 ACP. Sten and Thompson submachine guns were used as benchmarks. As part of the testing, all of the guns were immersed in mud and covered with sand to simulate the harshest environments in which they would be used. The Owen was the only gun that still operated after the treatment. Although the test showed the Owen's capability, the army could not decide on a calibre, and it was only after intervention from the higher levels of government that the army ordered the 9×19mm variant.