It's meant more as a way to increase the accuracy of the shots at range. So basically its more like a tactical sight than an actual scope.

The 180mm cannon used by the RX-79[G] Ground Combat Gundam can mount a opitical sight turning it into a long-range rifle (seen in Blue Destiny manga). The 100mm MG is similar to the M-14 rifle its only accurate firing one shot at a time and in short bursts of 3 to 5 shots. Its all about fire control...

If you look a little harder you notice that it is actually recessed into the hand so it would be a little harder to damage.