Yes it is possible to effectively kill ducks to 100 yds and beyond, but not with guns that are legal in the US today. Testing was done to prove both pattern and penetration was sufficient along with live shooting of ducks. This was accomplished with both 8 and 4 bore guns.
This is according to the book, "The gun and it's development" by H.H. Greener.
See page 396, "Killing range of large calibres"
This is some great stuff in here, the book is HUGE 838 pages so downloading takes a bit of time.
http://archive.org/details/gunitsdevelopmen00greerich
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