The special mode “Brittleness Coating” of the Bone Saw has an incorrect stacking count for its layers. Its light attack causes 3 stacks of “Brittleness,” or 4 stacks if it triggers a critical hit. However, it is impossible to stack up to 16 layers under any circumstances. The stacking limit is 15 layers, resulting in 37.5%, rather than the normal cap of 40%.
Switch Mode
Applies a different chem coating to the sawblade, cycling between the Chem Toxin coating and the Brittleness coating.
The max stack count is 15, not 16. Bone Saw weapon special applies a unique debuff (saw_rending_debuff), it is not the same as the regular rending debuff (rending_debuff).
Wait, does this mean that the two can be added and superimposed?
37.5+40=77.5%
Yes.
The GOAT always popping in to explain Fatshark’s game for them 
FWIW, Fatshark did mention details on the brittleness coating in their news post about the DLC class:
When using the Brittleness coating, the weapon applies on-hit a small amount of stacks of an unique Hive Scum Brittleness debuff, granting additional damage from any source against the more hardened armour types.
This effect has a limit of 15 stacks (for a total of 37.5% Brittleness), but can be applied in addition to other forms of Brittleness without sharing the same maximum cap.
