@MaskedMammal You have the right idea but your wrong. A bullet is the projectile of a round. Both shotguns and rifles use rounds. Older shotgun rounds where brass cased and some handguns had incased bullets, like the 7.62 nagant. A slug is a bullet, just like ratshot rounds could be called a she...
It's called heavy because it's mech. And part of a Heavy BCT.