This is not an easy task to accomplish, but as the battlefield changes, the difficulty of the unity work is becoming lower: the industrial system of the Dwarves is collapsing, and their supply of weaponry is decreasing rapidly.
If they can't produce, then they can only ask from others: Ice The First requests the Elves and Suthers to provide enough weapons and the ammunition that supports them.
The Elves only have Mauser rifles, interestingly, Suthers happens to use Mauser rifles too, with the same caliber as the Elves. Thus, the future Allied Forces weapon standard caliber was chosen as the caliber of the Mauser rifle.
When it came to submachine guns, things changed a bit: The German submachine gun used by the Elves is more expensive and less easy to produce compared to the Dwarves' PPSh, making it difficult to choose.
