I don't quite remember how this works (please tell me if you know

) but once you give a squad a weapon upgrade (like the scoped lee-enfield), any weapons that go into their weapon slots will have a chance to drop.
This is also why Guards Rifle will drop their dp-28's (they use weapon slots), but not their ptrs rifles (considered a weapon upgrade).
Edit: Nvm, ignore that last part.

I just checked and it isn't like that.
Edit 2: What the previous three posts said makes sense. Just ignore this one.
ferwiner wrote:
Actually guards have 4 slots and 4 weapons in these slots. What they drop depends on what was the model that just died holding.
You're right. For some reason I thought PTRSs worked like G43s and the scoped lee-enfields.