Mick-

Couple things on casting technique.

Get two pots of water going.
One for the double boiler, one for the barrels.

I've done scores of cerrosafe castings, and have found that it's very important to have the barrels up to temperature in order to get a good finish on the casting metal.
Casting into cold steel, even room temperature, will quickly freeze the casting metal before it gets a chance to pick up consistent bore features.
If you're using a funnel, preheat it too.

Also.
Although the metal is re-usable, be careful about what you're melting it in. It's easy to pick up metals and change the alloy of the cerrosafe, changing it's flow and melting temperature.
I use a clean pyrex measuring cup, nice too in that it's got a spout.

Did you get the big brick of metal?
If so and you have enough, may I have a casting of your rifle-choke and a bit of bore?
I'd be happy to pay.