Resin Mold, No Definition (RTV Silicone)

If you left too much Vasoline in the cavity it could indeed hurt the shape of the casting. Wipe out the Vasoline- enough will remain on the mold to act as a release. In fact, if you are only making a couple of barrels you may not need any release agent.  The hardened resin does not stick that well to the RTV.

  Also, you must be careful with air bubbles.  The best way is to bring the pour sprue up to the bottom of the casting- making the sprue J-shaped.  Then, the design of the vent tubes is important.  You must have a vent tube placed at the highest point in the cavity.  If the shape of the pattern is complex- probably not true with simple gun barrels- you must have a vent tube at each local high spot.

For a gun barrel, make the mold such that the barrel cavity is vertical when you pour, make the pour sprue J-shaped and entering cavity from the bottom.  Then a single vent tube up from the top of the cavity should be adequate.

In designing the mold, try to think like an air bubble.  How will you escape from the cavity when the liquid resin pours into the cavity.

Thanks for replying.

Is there any low-viscosity resins you'd recommend?

Also, RTV Silicone is really messy. What is the best way to clean it? 

70/25 TIN - SIL is what I use. I buy my silicone rubber from www.uscomposites.com This is the softest silicone that they offer. It's easy enough to use although they formulate it to be mixed by weight. I mix mine by colour (light blue) so that it comes out the same way each time. Silicone doesn't stick to anything so moulding with vaseline, or any other release agent isn't necessary to get a good result! All you need is something that won't get gummy when exposed to it, or any type of glue. This was developed using resin, but you can use epoxy with it as well. What is impeding your progress is the fact that your using vaseline with epoxy. This will keep the material that you're using from adhering enough to the surface of the mould properly to get proper definition, or shape as it's better known as. Try it without the vaseline. 

~ Cobra Chris