This feature has been removed with the update to Orion Prelude in August 2014. Before that update players could quickly reload weapons by interrupting the animation with a melle attack or dodge/roll.
Bug or Feature?
Edit
The quick reload has frequently been reported as a bug, but the developers intentionally kept it and did not regard it as a bug for a long time. Supposedly this was a homage to a bug or feature in a Left 4 Dead game.
