ATTENTION:
This may be related to 6657 and 7167 , however it's a different
approach and not really about tactical reloading etc.

In general I think that reloading a gun takes not enough time in ArmA3.
Have a look at this video:

http://www.youtube.com/watch?v=uZ2SWWDt8Wg#t=0m47s

You can see an LMG gunner reloading which takes around 33
seconds.
Now, the thing is that this mainly takes so long because he puts his
magazine back in/onto his vest.

So, here'S the Idea, as an example:

Default, slow
R to reload, the soldier puts the magazine back in his vest which takes quite a bit of time (~30 seconds for an lmg)

Alternate fast reload:
CTRL+R, reloads the gun BUT drops the current magazine which then lies on the
ground.

And, in general as mentioned above, I think reloading should take a bit longer.
Of course, there are plenty of videos showing people reloading an AR15 in 4 seconds but they just do a tactical reload while dropping the magazine...
you get the idea.