Turns out that the MK14, like any other semi-automatic weapon has a fire-cap. This means is has a limit in how fast it can actually shoot, regardless how fast you pull the trigger.
I tested this now and these are the results.
Without Rapid Fire
Clicking my mouse in high-speed to shoot it only shot every second/third click.
Basically, regardless how fast I clicked there is a limit in how fast it can actually shoot.
With Rapid Fire
Clicking my mouse in high-speed to shoot it shot every click.
When I tried really hard I actually manage at some stage to hit the new limit
I know every person clicks their mouse in different speeds and mine may not be as fast or as slow as yours.
The bottom line is that that semi-automatic weapons have a firing-cap that in the case of the MK14 seems approximately 2-3 shots a second without
Rapid Fire and
at least double that with
The default firing-cap, may or may not vary between different semi-automatic weapons but I have not tested that.
Rapid Fire increases that cap by about 100%.
If you are trigger happy and you are able to click that mouse with ease so fast that you find your semi-automatic is not able to keep up with you then
Rapid Fire is going to help you out.
If you are like me were it rarely happens then
Rapid Fire is not as much of a benefit to you.