CW Bullet I.D. Needed

[Ripcon]

Here's a small 3 ring fired bullet I dug from the battle of Port Gibson, MS.
I've never seen a 3 ringer this small before. I don't have calipers but it appears to be around a .45 calibre in size.
I've got a few i.d. books but don't see anything similar. Any help or guesses?  Thanks!

[Ripcon]

For comparison purposes, I've lined it up against a standard .58 calibre minie ball.

[Jim T]

Tennessee Rifle bullet.   See pages 185-186 in "Round Ball to Rimfire" Part Four.