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.
In accordance to accuracy related to shotguns, this will widen the spread of the pistol shotguns as well.
Damage increase value of this mod stacks additively with that of Hornet Strike to a total of +286% additive of damage.
The decrease in accuracy has no noticeable effect on the Embolist, however the center "beam" of the weapon (think of it as a Flux Rifle beam in the center) will wobble. The Ignis also behaves in this manner, where an "invisible beam" is directed at the center of the reticle, allowing headshots with these weapons, and this is what Magnum Force will reduce.
Targeting Subsystem can almost completely negate the accuracy penalty from this mod once its effects activate, down to a 3% reduction of accuracy when combined with a fully-ranked Magnum Force. Keep in mind however that the accuracy bonus from Targeting Subsystem will only take effect once you hit an enemy and will only apply when you're aiming.
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.
The Angstrum can benefit from this by allowing its charged shots to cover a larger area at the cost of limiting long-range capabilities.
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 to give weapons an accuracy penalty instead of a recoil penalty.
"Magnum Force" is the name of the second movie in the "Dirty Harry" series of films starring Clint Eastwood.