Why cant you get it working with particles? It would require a lot of small particles to look smooth, but I think it shouldnt be that hard...

BTW , from what I saw in the video, the particles are the only way, because the smoke changes spawn point as the barrel moves, but the old smoke is still hanging somewhere lower than the barrel during reloading. This can only be done with a trail of particles left from the movement of the barrel.

Get the vertex you want to spawn from, use an effect with lots of small particles (somewhat smooth at the edges, but not too smooth) and set them with a very, very slow upwards movement and medium fading, to create a visible trail effect. I think this should look similar to the video...