Action and Reaction is one of the best early-game Weapon Modules that you will get to acquire in The First Descendant. It’s of the rare variety, specific to General Rounds, and you will be able to use it on handguns, machine guns, submachine guns, and assault rifles.
The Module will help you to increase your firearm attack while lowering the recoil by a certain amount. Making it a great option for weapons like the Tamer.
This is especially helpful in the early stages when you are yet to start minmaxing your loadout, and you will need all the DPS that you can get.
Here is a guide on how you will get to acquire the Action and Reaction Weapon Module in The First Descendant.
How to farm for the Action and Reaction module in The First Descendant

To get your hands on the Action and Reaction Weapon Moduel, you will first be required to make your way to the Sterile Lands. This will happen only after you have been able to complete all of the main missions in Kingston and defeat the Grave Walker Colossi.
Once you are in the Sterile Lands, keep completing all the story missions. You will eventually make your way to the Classified Area which is the final region there.
You will find the “Large Nuclear Reactor” mission which has a chance of dropping the Action and Reaction Module upon completion. As it’s in the very early stages of the game, you can use a Descendant like Bunny to keep restarting the mission and farming for the Module.

On Normal difficulty, the Action and Reaction module has a 12.5% chance of dropping from the mission. However, once you complete all the missions in the Sterile Lands and unlock Hard mode in the region, the drop rate goes up to 30%.
So if you are target farming the resource and not having much luck in Normal, switch to hard to have a much higher chance of getting your hand on it.