My questions is, does the rapid fire suit module work with marshall, DSG, Gauss, Takedown, and Grendel?

Yes, try it the Nova it's a beast!

Yes, the Rapid Fire module works on Gauss,Grendel,DSG pretty much all weapons except the bow. In fact, Rapid Fire 3 with Gauss makes the weapon bearable.

Does it works with the .50cal?

so if i put rapid fire on the takedown its going to make it auto instead of semi auto?

so if i put rapid fire on the takedown its going to make it auto instead of semi auto?
No

so if i put rapid fire on the takedown its going to make it auto instead of semi auto?
Err no lol that's not what it does.

Rapid Fire decrease the time between each bullet by the specified %. So with Tier 3 the delays between your Takedown shots are going to be 25% shorter i.e. you can click a lot faster with it. But you'll still have to click for each shot, it's not going to turn into an automatic gun

Yes, try it the Nova it's a beast!
I don't recommend trying this unless you enjoy getting Guitar Hero syndrome, or are a huge fan of Diablo games.

Guitar Hero syndrome,.
????

Guitar Hero syndrome,.
????

funny s,hi,t............