Instead of reducing the accuracy by 3% per rank, each rank increases spread by a specific percent per rank depending on the weapon. This effectively reduces accuracy but may reduce it by greater than 3% per rank (according to the Arsenal UI).
For example, a rank 6 Magnum Force will reduce Acrid's accuracy to 4.3 from 100.
This mod is generally inefficient, with an extremely low damage bonus compared to Hornet Strike, an accuracy reduction, and a prohibitively high cost both to level and to equip. It is currently considered totally inferior to elemental mods and faction mods for raw damage output.
When it was first released, Magnum Force increased recoil as well as damage. However the recoil penalty did not affect recoil-less weapons (Acrid, and Embolist, among others). This was confirmed to be unintentional in Warframe Prime Time - Episode 2 (9:40). The mod's effects were changed Update 10.4: it now gives an accuracy penalty instead of a recoil penalty.