Sorry, i meant cylinder. :headbang:

Just create a cylinder with about 8 segments running around the axis. Position this at the rear end of the barrel. This will give it the cylindrical shape you want. The extrude the cylinder forward toward the front of the barrel.
Now bring in the 2 side edges of the section you just extruded, so that they're flat. This will create the boxed section at the front of the barrel.

So now you should have a flat side that leads of to a rounded side.

Then it would simply a case of adding edges and tweaking them into the right shape.

That image you posted of the Gun is a good side view, you should load it as an image plane.

If this doesn't make any sense, I'm sure someone else will be able to suggest a better approach.

The best thing to do is load up the your image plane(s) and wing it! Just start experimenting with different ways of accomplishing the shapes your after.

Odds are something will come to you, and you won't have only modeled your gun, but you also learned allot more along the way by going at it yourself, than following a step by step procedure.

Well thats about all I've got at the Mo.

Tomorrow I'll be able to get on my computer, and maybe I'll give the gun a go!

Good luck!