You may be tempted to place the original in the mold VERTICALLY, so you can poor the resin from the back end and have a smooth barrel.
That probably won't work, unless you do have a vacuum machine. No matter what you try, there will always be air bubbles and warping...
Without vacuum, I would suggest you make a HORIZONTAL copy of the barrel. To achieve this, you place a piece of rod (square is the easiest) on the bottom of a container. Next, you add a very thin piece of plasticard strip on top of the rod. Both the rod and the strip should be the same length as your barrel. Finally, glue the gun barrel on top of the plastic strip, also lengthwise. Seen from the tip of the barrel, the whole construction should look like this :

o (gun barrel)
! (plastic strip)
O (piece of rod)
_ (container bottom)


Now you poor the silicone over it, covering the whole construction.
Once hard, remove the original. Turn upside down and poor resin.
The advantage of this form is that you can "help" the resin reach every little corner of your barrel without leaving air bubbles. You can help this proces by moving the soft side of a pin (head) forward and back inside the barrel while the resin is still liquid. Because of the strip that was glued to the barrel, you can hold the mold "open" to poor the liquid and to scrape out the air bubbles. Once hardened, let the barrel inside for at least an hour till the resin is very hard. That way, it won't warp so much. Now remove the barrel. Because of this procedure, the strip will result in a flash along the whole lenght of the barrel. You can remove this flash with an x-acto and sanding paper.
That should do the trick.

If at first you don't succeed.... try Armorama !

Jan