Stoney made a very good point with the measurement comparison! Correct he his, .50 calibre and maybe even .54....hard to say with the naked eye but definately not .32 calibre. A penny measures real close to 3/4 of an inch (.750 calibre). My "eye ball" measurement of the bridge of Lincoln's nose put that measurement at a solid .50 if not more. Looks like the bridge is a little higher than the "center" of the penny. Dead center of the penny would measure .375 calibre.
At a depth of 5.5 inches, I'm willing to bet the farm that it is 150 years or older. In my soil conditions, the more mass an object has, the slower it will sink. I have found large Civil War camp bullets at 2.5 inches and found .32's and .22's on the same camp at 7 inches. The majority of the modern balls that I have dug are 1.5 inches or less with a very light, see thru type white patina. I'm certain that soil conditions play some role in the process.
In the State of Kentucky, if it quacks we call it a duck.....congrats on finding yourself a Civil War (or older) bullet.