Again, just enough energy left for one reply-post tonight.
According to the rifling-grooves list on page 169 in the McKee-&-Mason book on civil war bullets/projectiles, there were only two kinds of .44 pistols which had 7-groove rifling, which is what we see on your bullet in the base-view photo. Those two pistols are the .44 Colt and the .44 Tucker & Sherrod. The latter was a rare pistol, while at least 100,000 Colt .44 pistols were used in the civil war. So, the odds heavily favor your pistol bullet having been fired from a .44 Colt. The short-looking bullet is the correct length to be from a Colt New-Model bulletmold.
I must also mention, that's a really major loading-lever impression on your bullet's front end. The pistol's cylinder must have been heavily "fouled" with gunpowder ash from nonstop all-day fighting, causing the pistol's owner to have to use great force to press the bullet into the cylinder.