Thought while I was waiting for the glue to dry on the transom planking that I'd start doing some gunport frames. I thought shouldn't be a problem on a little fiddly.
I started as the instruction said by checking the size of the first porthole. Yes 12x12 exactly. So i then proceeded to cut the side frames and place them. Perfect fit. Then I decided to do the top and bottom frames. I referred to the instructions again and noted these should be 9mm, so I cut them accordingly. I was surprised to find they would not fit and seemed to be to long. I checked the measurement between the side frames only to find that the gap was in fact 8mm. I then checked the thickness of the supplied wood and sure enough it is 1.5mm. So,1.5+1.5=3 , 3-12=9. So, my question is;why am I getting a measurement of 8mm?

I agree with Steve, your framing size is going to be comparable to your gunport cut out size, best to have them a tiny bit long to make them a slight force fit, no problems with gaps or gluing then.

Wilfy