It seems like most of the jams are caused by using any shape of pellets other than wadcutters... wadcutters after all are the only shape of pellet that sig says to use. So why use any other shape??
I've only had one jam in my mcx in a couple thousand rounds and I'm positive it was due to the fact that some pellets fit loosely in the pellet belt(the jam in question was caused by Excite "plinking" wadcutters), allowing them to wiggle forward during operation and cause a jam.
So try and stick to wadcutters that fit tightly in the belt and you should have fewer issues, I cant say for sure if this will fix all jams, but I think it'll fix most of them. For reference, crosman wadcutters have a nice tight fit. I hope this helps any of you that may be frustrated with your mcx/mpx rifles.