Looking through the weapon files, I found drop rates. However, any weapon that is capable of dropping (i.e. PTRS rifle which is droppable vs G43 which is NOT droppable) has a .33 drop_rate, whereas all other items have a 0 drop_rate.
| slot_item_bag: {
| | always_pick_up: false;
| | can_stack: false;
| | can_stack_max: 1f;
| | drop_item_entity: "soviet_guard_ptrs_mp";
| | drop_rate: 0.33f;
| | hardpoint: 1f;
| | has_speech_code: true;
| | is_entity_user: false;
| | item_uses: 0f;
The Panzershreck is the only item that has 3 seperate entries, in which all other droppable items simply have one entry.
Panzershreck has:
panzershreck_mp
panzershreck_slot1_mp
panzershreck_slot2_mp
Again, the other weapons are simply weaponname_mp, which might explain if shrecks really do only drop when they are at 2 models or below; though anecdotally I can't say if this is true or not.
If there aren't other mechanics at work, ALL droppable items have a 33% chance to be dropped on model death, and a guard squad has the most items out of all squads in the game that can be dropped.