The ball appears to be unfired. It is the precisely-correct diameter (.445") to be an unfired pistolball for an 1840s-60s Colt Army-model "cap-&-ball" .44-caliber revolver. Its bullets are larger than the gun's bore-diameter because they are loaded by being pressed into the revolver's cylinder, which is located at the back end of the gunbarrel.
We need a couple more sideview photos of the cylindro-conical bullet, showing its "other side." The single sideview photo you posted seems to show at least one body-groove on it, maybe two, and I'm wondering if more than one shows on the bullet's other side.
Being fired means your .420" diameter measurement of it includes the height of the rifling-grooves. So, it is a .41-caliber bullet, and could be for either the 1880s-90s era .41 Colt revolver, or the latter-1870s-80s Fuller-&-Fulton, Smith's Patent .41 revolver.