While attached, a heavy barrel reduces the cone-of-fire bloom of the weapon while aimed by 20%. In exchange, the weapon's aimed movement speed multiplier is reduced by 20% down to either 0.4x or 0.6x, for weapons with a base multiplier of 0.5x or 0.75x respectively.
This attachment has no effect on any hip accuracy characteristics, nor does it affect the minimum cones-of-fire of the weapon while aimed.