If you play outside with an M4/version 2 mechbox in the winter, you are just looking for trouble. Version 2 mechboxes are notorious for breaking in the cold with stock springs. It's an inherent flaw in design, and it's not even fixed by so-called "reinforced" mechboxes, which have all their reinforcement at the gears (where it's redundant and unnecessary) and not at the front of the mechbox, at the cylinder window (where it's actually required). You can try to alleviate some of the stress on the mechbox by radiusing the corners in the cylinder window, but even that is not a proper fix.
If you intend to continue playing outside in the winter, I highly recommend you invest in an Ajax Customs Shock Transfer System: