In a colony 50 years into the future with few metal resources, but large population on a relatively densely forested (forests similar to those of north america) island similar in size to greenland is about to be invaded by a foreign power. The islanders realize they are about to be attacked, without any outside help to save them. They decide they must arm their population with weapons to neccessitate a guerilla war if their relatively weak military fails. Their problem is that they must create compact assualt rifles or submachine guns using as few moving parts or metal components as possible without compromising too much combat effectiveness. What weapons technology/ designs would be usable in this situation?
Conditions: The weapon must be selective fire (semi or full auto, or semi or burst), it must be compact (carbine size or shorter), it must be very reliable and optimized to work in wet/ muddy conditions, along with cold winter conditions, Easy to manufacture, mid to short range weapon, use as few metal components and moving parts, easy to maintain/repair, decent combat performance, it should be chambered to fire either a 7.62x51mm NATO, a 5.56x35mm NATO, or a 9x19mm if going for smg.
EDIT: this country has a limited industrial capacity based entirely on rudimentary manufacturing equipment sent in with the colonists, before these same invaders. They do have access to a few metal 3d printers, and some plastic ones (limited plastic), they have lots of wood working equipment also and lots of spare parts for each. They also have a mill, lathe, and a drill press. Along with blow torches with limited fuel for welding. They do have a mine, that does produce some metals. They rely for power generation from a wood burning generator.