Man.. i have not even been able to play yet but already knew how this worked!
Use one consumable and kill the Ultra.. if you don't get a drop, use one again and the percentage will be higher for a drop to happen.
Rinse and repeat as much as you want until you get the drops you want. ONE AT A TIME!
Stacking 5-10 or whatever does not increase the chance on that run! It has the same effect as using just one coin!
Don't waste them people!
Edit - after using around 70 three of coins one by one , I have now got all of the new guns. Missing just the heavy year 2 weapons. Done well with armour to although had the Stag and Alpha Lupi 4x each!!! I am getting an engram drop around 5-8 goes each time.
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I heard this also.