All M4's use a version 2 mechbox....you need to replace it with a version 2 (version 3's are a different shape)
to fix it:
1. Buy a replacement v2 mechbox shell
2. swap the internals and everything else into the new shell...reshim, etc...fingers crossed it works. Down grade spring to m90.
3. Break that one in the cold as well and realize that you'd be better off with a V3 gun for the winter...or a PTW if you have to have an M4. (just kidding)