to cast or not to cast?
if your thinking on doing castings for say a 20 gun model it is not worth the cost and the time. To start with it is really difficult to do casting without a vacuum pot, yes it can be done and if your good enough at it you might get bubble free molds and castings. One issue is you will waste a lot of time and materials before you get results. In the end those cannons may cost you $30.00 each, the cost of the molding rubber and resins are expensive.
Other than casting there is CNC but very few model builders have a CNC lathe or the software to create the cutting files so this is pretty much out of the question.
another method is 3D printing here again very few model builders if any have high end printers or are able to create the print file. If you can create the print file you can send it to a printing service. most of these services are low quality and the high end printers are to expensive. A 3 inch long cannon can cost as much as $60.00 the print looks fantastic and the detail is perfect. This is used to print one master and thus the use of casting.
here are some examples of 3D printed carronades and you can see what the quality is. Some are from places like Shapeways others are from other services. Now I have not tried all the different materials offered so these are an average print. I am sure the more you pay the better the quality
different materials produce different results
different resolutions produce different detail quality
different material different results in detail
depending on how much your willing to pay will give you sharper prints
you get what you pay for a cheap $8.00 print looks like this
middle of the road cost for a print is usable still expensive but passable.
to cast or not to cast? pay for a high end high resolution resin print and make a silicone mold and cast the cannons