The additional rounds do not consume additional ammunition.
At rank 4 or higher, this mod guarantees that at least 2 rounds will be fired for every 1 round of ammunition expended.
At maximum rank, this mod grants an 80% chance to fire one additional round (2 rounds/shot) and a 20% chance to fire two additional rounds (3 rounds/shot).
Equipping this mod and Lethal Torrent simultaneously will provide 180% multishot, giving the sidearm a 20% chance to fire one additional round (2 rounds/shot) and an 80% chance to fire two additional rounds (3 rounds/shot).
For weapons that fire multiple pellets (i.e. a Bronco) the pellet count is multiplied by the multishot value.
A Bronco with a rank 5 Barrel Diffusion will fire $ 7 \times (1 + 1.2) = 15.4 $ pellets, meaning it has a 60% chance to fire 15 pellets and a 40% chance to fire 16 pellets.
The in-game UI will consider any possible multishot capabilities when showing total damage numbers, and does not represent single-projectile damage.
Total damage displayed does not represent the actual damage per shot, but the expected average damage.
Equipping this mod increases the status chance of a weapon indirectly. This is due to the possibility of shooting more than one projectile.