Thank you dugfinds, trk5capt, & romeo.
"As originally designed by Minié, the bullet had a small iron plug in the base whose purpose was to drive forward under the pressure of powder gases to fill the hollow space and expand the lead skirting to grip the barrel's rifling. As finally adopted by the United States Government before the Civil War, however, the skirt of the bullet base was made slightly thinner and the plug omitted, as the pressure of the powder gas alone was sufficient to expand the skirt to engage the rifling".(from Wikipedia).
I can see now why some bullets were blown out clear through.
I am surprised to find something that old on a South Florida beach only 4 feet deep. My bullet has no grooves. Does this mean it was a smooth bore? Can anyone ID?
below pic is from dugfinds.