I recall an article written several yeas back written by Wallace Labinsky (not sure of the spelling) in which he tested a variety of different (Lead) shot all from the same gun & in identical loads. For some shot which was not available to reloaders he opened factory shells & used the shot only. His highest percentages were obtained using unplated premium hardened shot, Diamond as I recall. This shot produced higher percentages than any of the plated shot, either Nickel or Copper.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra