What feed spring did you use? I have a Rap4 T68 mag here that I tried the spring out of, it's too large in diameter and too long. Easy enough to cut it down, I don't know if I'll get away with trying to wind it smaller.
I can tell you the rear plug/velocity adjuster is not the same as a Spyder. I tried a metal Spyder plug to further remove more plastic from the thing, doesn't fit at all. The donor Piranha has a thumb wheel adjuster, but has rear cocking so the whole assembly is a little different.