[MadMax]

When you say that you do not have to thumb the bolt release when changing mags, does your PTW stop cycling on an empty mag an automatically resets on an full mag? If your PTW has ceased firing on an empty mag and you remove the mag, will the mechbox cycle right after removing the mag?

Try this other test:

Remove the mag catch and hold the magazine into the receiver by hand. I would say that EVERY SCK built PTW that I have seen required some degree of mag catch modification to assure good registration in the mag well.