More-specific identification:
Your bullet is a Williams Patent .58-caliber "Bore-Cleaner" bullet, from the civil war. It is the third version of his patented bullet, so it is known as a "Type 3" Williams Cleaner. It was first manufactured in 1863, and starts showing up on civil war battlefields in late summer of that year. The Type 3 was used extensively in all theaters of the war east of the Mississippi River in 1864 and 1865. All Williams bullets are yankee-made, and all are .58-caliber.
In actuality, according to Mr. Williams' US Patent, his bullet's unique design was intended as an improved way to get the bullet to grip the rifling-grooves in the gunbarrel... NOT as a "bore-cleaner." But the thin cupped (bowl-shaped) zinc washer (between the flat hardened-lead disc and the bullet's base) did have the side-effect of scraping burned powder residue out of the rifling-grooves as it traveled through the gunbarrel upon firing, so it picked up the nickname "bore-cleaner" bullet.
Firing the bullet pushed the flat base-disc (a.k.a. "Williams thumbtack") flush up against the bullet's bottom, compressing the thin bowl-shaped zinc washer outward into the gunbarrel's rifling-grooves.
The thin bowl-shaped zinc washer is almost always mostly or entirely corroded away on excavated Williams Cleaner bullets. That is why there is a .10-inch gap on your unfired bullet between the base-disc and the bullet's bottom.
Soldiers hated the Type 2 and Type 3 Williams bullets, because the flat hardened-lead disc inserted into the bullet's base when it was manufactured tended to pull loose and jam in the barrel when you tried to unload it out of the (muzzleloading) rifle. That is why we relic-diggers find very large quantities of UNFIRED Williams Cleaner bullets on battlefields. They originally were issued at a rate of one in every ten rounds when the soldiers were supplied with ammunition... so the soldiers kept the nine "regular" bullets and tossed the Williams Cleaners.
One of the photos below shows an unfired Williams Bore-Cleaner Type 3 with its bowl-shaped zinc washer intact, although a bit corroded-looking. The other photo shows the "base thumbtack" disc pulled out, and mostly pulled-out.