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.
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 reticule, allowing headshots with these weapons, and this is what Magnum Force will reduce.
There currently are no mods to counter the lessened accuracy of this mod.
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.
Angstrum can benefit from this by allowing it's 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: it now gives an accuracy penalty instead of a recoil penalty.