Originally Posted by GBBR
they are not
actually the slide and frame is compatible, with the 17 not sure about the 18 but they should be similar since they are both based off the marui system. Buddy of mine fitted a Gaurder Frame and slide onto his HK3P 17 which is esentially the WE since its OEM HK3P.
Internals should be similar to a certain extent if the frame and slide fits.