Use a program to convert the HUD.x file for the gun - located in the gunbank folder - into a model format of your choice (I'd use .ms3d but that's just because I use Milkshape). Open up the model in your modelling program, select those vertices, delete them, patch up any holes as best you can by moving the vertices you have left over (or alternatively remodel the entirety of the scope mountings and UV map that part seperately, and then assign the new mesh you've created to the main bone for the gun - this is complicated). Save the new model, back up the old HUD.x, open up the model in Fragmotion, and export the new HUD.x file back into the folder, animations included.
It's quite a complicated thing to do.
