The bore in my 1779 naval cannon is 14mm diameter, 270mm deep.
I made a D bit from silver steel, as per the Jerry Howell plans. I tried it without heat treating, but it blunted after boring a couple of centimeters so I heated it red hot and quenched it in water, then annealed it and resharpened it. There were no further issues with edge holding.
I then tried it without, then with, a preliminary drilled hole in some scrap. I have decided that it is better to give it a starting hole of the correct diameter.
The 270mm bore took 2 hours to complete. It was not a boring job. I was anxious not to muck up the hefty lump of brass.
Next to drill the trunion holes in the barrel stock. That will be straight through all layers of the barrel. (retrospective note added later… The trunnion holes were stopped short of the bore, and I was just very careful to keep the holes at 180 degrees and in line)
Then to turn the exterior of the barrel. There will be a video if that is successful.
Then to silver solder the trunnions to the barrel in one piece. Then to use the D bit to rebore the barrel, removing the trunnion rod which is obstructing the bore. Some readers will not agree with this method, and it is not according to the Jerry Howell plans, but it does ensure that the trunnions are exactly in line with each other. Silver solder, if properly used, is said to be as strong as the parent metal, so I believe that I will not be compromising the integrity of the barrel. The main disadvantage is that the finished exterior of the barrel will need to be held in the 3 jaw chuck during that final D bit reboring. I have not quite worked out how to do that, while avoiding marking the finished brass surface.