I have the G&P M203 Long version. If you use the BB's in the shells the effect is more like a large shot gun. But I Prefer to use the howler. This is similar to what a real m203 is used for. With practice and timing they can sometimes be pretty accurate. Accurate in the sense that there is a “blast radius†associated with the howler. The “radius†is determined by the game organizer, but usually is between 10 to 20 feet. Just like the rest of Airsoft, this is done on the honor system, and we all know that is what keeps this sport going and growing.
On a side note, there was this one time in Prescott that the radius did not matter for me since the round hit my target in the forehead. Like I said earlier “timing†is important.
Just to let you know, Nerf no longer makes the “Howlers†that are used. This makes acquiring them very difficult. They still make the small football, but these do not make any noise as they are coming in on target. Which for me is what makes carrying the entire set up worth while.
When using the Howler, they must be stuffed down the front of the barrel and the shell, with the gas inside, is breach loaded as normal. The pressure inside the barrel is what causes the Howler to launch. I estimate the Howler can travel near the 75 meter mark, but I have never actually measured the distance.
I hope this helps answer your question.