Opinions please

[JNimons]

This is kind of a double post. If I'm breaking the rules, feel free to delete. I found what appears to me to be shrapnel of some sort on one of my farms. I have found 3 pieces, but discarded the first 2. I recognize them by the small ribs. It appears that there is rifling on what is now the inside of the piece.

http://metaldetectingforum.com/showthread.php?t=226206

any ideas?