Instead of reducing the accuracy by 5% 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 5% per rank (according to the Arsenal UI).
This will increase the spread of pistol shotguns.
Damage increase value of this mod stacks additively with that of Hornet Strike and Augur Pact to a total of +475% base damage.
Accuracy reduction varies by weapon, even for those with 100 accuracy. For example, Acrid (which has 100 accuracy, decreased to 1.6 at max rank) needles fly very far outside of the reticule, while Akstiletto (which has 23.5 accuracy, decreased to 10.3 at max rank) spreads bullets considerably but don't stray too far from the reticule.
The following weapons below are unaffected or barely affected by the accuracy penalty:
Targeting Subsystem can help negate the accuracy penalty from this mod once its effects activate, down to a 25% 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.
If a pistol already has a max rank Hornet Strike, adding a max rank Magnum Force represents only a ~54.6% increase in damage per shot, compared to a 90% increase for a max rank uncommon elemental mod such as Heated Charge.
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.
This should only be used on weapons that already have Hornet Strike installed, as the bonuses gained from this mod are greater than Magnum Force and comes with no accuracy penalty. Also take Augur Pact into consideration as it costs less mod capacity without the accuracy penalty.
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.
Prior to Hotfix 24.2.10, Magnum Force provided +66% damage and -33% accuracy at max rank.