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 will increase the spread of pistol shotguns.
Damage increase value of this mod stacks additively with that of Hornet Strike to a total of +286% base damage.
The center "beam" of the Embolist, and other continuous weapons 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 make less accurate.
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.
If a pistol already has a max rank Hornet Strike, adding a max rank Magnum Force represents only a 20% increase in damage per shot, compared to a 90% increase for a max rank uncommon elemental mod such as Heated Charge. Even with three other uncommon elemental mods, it is more efficient to add the fourth (and get +24% damage) than it is to add Magnum Force.
This mod also fixes the inaccuracy bug that occurs when using multishot on the Castanas and Sancti Castanas at low level, but it's still advised to max the mod anyway for extra damage and higher spread on the Castanas, leading to more crowd control.
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 in 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.