I have this same problem and cannot get the Big Sticks to run. I'd love to see some pictures of the modification you have done.
Even the 140mm mags with CZ internals are only reliable holding 20 rounds. I think after 20, that the spring catches on the sides under the control ribs.
I'll try to get some more details up soonSent from my SM-G900V using Tapatalk
Any update on this by chance? I am having the nose dive issues with my czechmate.