I'd like to pipe up here and say MP5.
They are relatively lightweight, especially the ones with plastic uppers, and more importantly, you can carry 2 MP5 magazines for the weight and space of 1 M4 magazine. An MP5 with collapsible stock and 10 mags would do you well in my opinion. They fair quite well outdoors as well.
Best of luck.
Ár skal rísa, sá er annars vill
fé eða fjör hafa. Sjaldan liggjandi úlfur
lær um getur né sofandi maður sigur.