Shortround6
Major General
You basically need a box. How strong the box is made may be a question. The Belt feed does not have spring forcing the rounds to the gun. The drum has to guide the rounds to the gun and keep them properly aligned. The belt needs just enough container to keep things pointed the same way while maneuvering, but pulling the belt helps (does not eliminate) small problems with alinement.But there is a catch. You always need some magazine even for the belt, that also has its weight, and drums were extremely convenient as self contained structural elements. The belt also has its non-zero mass. Therefore, the additional cost of the magazine mass in relation to the belt was much smaller.
Try pushing a few dozen tapered pens along a table and see how well that goes
Granted the belt feeder/de-linker was about 8kg for the Hispano but the weight didn't change much, if any for long belts. Weight of links was certainly there.
Most magazine fed guns had covers/doors over the magazines so there may not be much savings in weight there.
Large magazines/drums soon got unwieldly.