OldBob, there's no way the base of a ball can be flattened inside the barrel, unless perhaps there's a wad between the powder and ball, or the ball was pounded mighty hard against the powder charge in the loading process. With nothing between the ball and the powder pressure is equal on all parts of the sphere exposed to the charge.
I've recover numerous pure soft lead balls in .50, .54 and .58 caliber from the sandy loam behind the 50 yard target frame on our rifle range. Unless they struck another bullet in the soil upon impact they remain almost perfectly round.
I've only recovered one ball from a deer (all others were pass throughs). It was a .535 ball shot into the chest of a 155 lb buck on a head on shot. The ball stopped against the inside of the rib cage, stuck between two ribs. It remained round and you can see where it impacted the side of a rib.
Here's that ball.
