What are the chances for mythic weapons in the end reward chest?

Hey ya'll,
I've been wondering how big the chance difference between the mythic corkian augments and an actual mythic weapon in the end reward chest is. Since I get the augments on a regular basis (about 7 now) and I have only ever gotten one mythic weapon from the end reward chest, there seems to be quite a big difference in chances. But how big is it actually? Does anyone know this? Or do I just have cursed luck?