11mm Pin fire
I am in the process to make a translation of a book from 1932 with detailed thoughts and insights of why the 8x63 were developed, but it takes time for me. The basic things is mainly what many persons already guesses about, the choice of calibre and the case length. Trials had been committed during 1905 to 1932 to find the best and suitable calibre to fulfill the needs of a new machine gun. The trials stretched from the 6,5x55 with a spitzer bullet and up to a 9 mm projectile. The 6,5x55 were too weak to live up to the needs regarding energy at hit. The 9 mm were too powerful, and to expensive to start production of as there were no tools in country and no machinegun developed to make use of the powerful cartridge. The tests were first in favour of a 7,2 mm projectile when looking at the accuracy and terminal effect. The withdraw of the calibre is that the estimations were that a calibre of 7,5 mm or larger is needed to give room for developments of new types of projectiles. It is not disclosed what projectiles the statement referred to, but I believe it is incendiary and explosive projectiles needed mainly when the machinegun is used as a ground to air system. The conclusion was maid that a 7,92 mm projectile were better aligned with the task at hand (influenced by the learning from Europe). After a redesign of the 7,92 projectile the performance was fulfilling the expectations.
The need for a machinegun were found within reach, the Colt 1919 used at the air force. It was already redesigned to use the 6,5x55 cartridge, so a new case was designed using the same bottom diameter as a 6,5x55 cartridge and the same case length as the American 30-06 that the 1919 were originally manufactured for. Now the best of worlds was gained. The 1919 were rebuilt with the mantle from a m/14 machinegun and were renamed m/14-29. It could use either the 6,5x55 ammunition, or after a quick re-barreling it could use the very fine 8mm full powered ammunition with a better variety of projectiles. In the end it took until 1939 before anything but ball ammunition was offered to the troops.