Well, I took some of your advice rollingb and just modified the existing nipple so it could take an 11.
Carefully used the diamond tipped grinders that come with Dremels/Wens and slowly removed metal around the nipple until an 11 just snugged on but its still too thick for a 10.
Fired the 11 and got a nice plume through the barrel.
Also cleaned up the base a little where it has the tool slip marks.
I looked down the barrel a bit and see orangey rust coating the length of the barrel and I've run patches through it recently. Most of the corrosion seems to be in the combustion area. Sprayed some more Hoppes lubricant down the barrel and then stood it on its barrel to drain back out.
Now I need to fit the right balls. ID is right about 18mm which converts to 0.708 cal. Pretty Big!