First, don't plug the barrel because that's the best idea for where to pour the resin inside.
Second, spread a light coat of resin over the outside of the weapon to seal it and let it dry.
Third, pour some resin inside through the barrel, place a piece of tape over the end of the barrel, and rotate the weapon around so the resin coats the inside completely. Remove the tape before it dries.
Fourth, pour rondo inside through the barrel and rotate to make the weapon solid.
Finally, bondo the outside and sand it smooth, adding whatever details you want.