The rotation of the YEHW is gone if that's what you think is a bad design. Since it's a front loaded single shot it does seem to be more like HYPR in that aspect. Elastic might work for elite caliber, but Mega XL needs more power like K25 spring. Also it needs the 2" wide plunger tube volume of the YEHW to get that Mega XL going.
There's a new gunslinger in town and he goes by Ruppernutter on thingiverse. This remix uses some of the original hardware and none of the prints.
It can fire Mega XL at 55 fps average