Cadillac Bumper Bullets are most often mid-size breasts that are far too tightly reined in by a badly designed or poorly fit pushup bra, that has them squeezed into a shape very much like a rounded cone. Commonly found on cougars trying to fight gravity, but occasionally on a younger girl that wishes she were bigger across the bosom. It lends an almost comical appearance.
Exceptions to this are sometimes found in the case of a very young girl of around 17 or 18, who is getting very buxom very fast, and her breasts seem to reach straight out from the shoulder...these are more correctly called sweater bumpers, but when the girls breasts are extra-large the terms become interchangeable.
"heheh, check the Cadillac Bumper Bullets on the cougar in the leopard skin, betcha it took half an hour to get that thing on"
"that little redheaded chick Robin is some looker, isn't she? and what a pair of CBB's she's got"