thanks Allanyed, this is helpful info.
(long answer warning!)
My first attempt at building a model ship has ground to a halt at the 2nd planking stage. Starting with what I though was a relatively easy project - Caldercraft's HM Cutter Sherbourne, I suspect my hull shaping went wrong at some point during a frustrating first planking (and heavily 'filler'ed') stage. This has resulted in bad 'clinkering' of the planks on both sides at the bow curves from the bulwarks down, which gets progressively worse with each sucessive plank. I also now realise fitting the planks while damp from soaking was also a mistake as several have shrunk when dry leaving even more gaps. With 2nd planking is about 50% finished, I'm certain the end result will be horrible, so have kinda lost the will to carry on with it.
Given that all I really wanted to begin with was a decent looking model ship for my bookcase, I've decided to try Costructos 1:85 St Helena Schooner (Sexyauthors build log helped make my final decision). This model has a solid hull (no planking!) and so I hope will be an easier project.
Rather than depict the St Helena ootb, I plan to add extra detail and a different paint scheme in order to depict a typical Royal Navy Schooner of the period - hence my enquiry about carronades.
Thanks to the responses here and some further research, I think about 4-6 carronades would be the most likely armament. If a 12 pounder was 26" long then you are correct that at 1:85 scale a 12pdr carronade would be about 0.3" or just under 8mm. Therefore it would be my indention to cut a 15mm cannon barrel down to size and construct my own carriage/mount.
thanks again for your info. Its exactly what I was looking for.
S.