Pellets recovered from mallards at 45 yds. Ed Lowry, "Shot Penetration in Soft Targets", American Rifleman, Oct. 1988



Probably not worth the $25, but it appears "shot packing factor" is the issue
https://www.astm.org/DIGITAL_LIBRARY/JOURNALS/FORENSIC/PAGES/JFS11154J.htm

Bob Brister's illustration
http://www.classicshooting.com/blogs/com...easant-shooting