You might want to know that some links are specific not just to caliber but to the machinegun as well.
Current US General Purpose machineguns use links which allow the round to be stripped off as the bolt pushes the round forward. However, the M2 (50 caliber Browning) and the earlier M1917 & M1919 pulled the bullet out through the rear of the belt.
In addition, for the M1917/M1919 series there are three different types of internals for the top cover - 1) for only cloth belts, 2) for only "disintegrating" metal links, and 3) for either of these two.
There is a lot to learn and understand about the technology - particularly if you will be looking at any of the older "legacy" designs. A trip out to a Knob Creek machinegun shoot might be a good investment for you to meet some of the more knowledgeable people in the field.