This is what I came up with while doing the effects for the normal gun. I went through a couple of tries before setting up with the best one.
The first attempt was done a bit through imagination and by messing with the particle system a bit. The second attempt was done as well by changing some particle system options and by trying to create a more explosion type of effect.
The last attempt, which resulted with a bullet and a smoke effect, was a result of the feedback given to me, after I showed the first two effects to the team. I did a bit of research into the particle system as to find out how to do it. I went ahead and used a sphere for emmision, with a radius modified to obtain the smoke effect, with some changes as to be seen like the shot of a revolver. Afterwards I parented another particle effect system to the smoke one, as to create a bullet effect. I did give up however on the bullet effect, as I am thinking that it could be done by a raycast while programming, therefore just leaving the smoke to appear when the shoot button is pressed.