Tony,

Your concern makes sense to me but, when the gun was a pinfire, that was just a hole in the breech end of the barrels and the gun worked fine. I would expect that there would not be much pressure at those holes because the holes are over the thick base of cartridge case.

I have read very little about changing pinfires to centerfires. I thought that either the barrels were set back slightly or the breech face lengthen slightly.

It would be interesting to see the barrel patches. Are they welded in place?

The pictures of your Howder in your article look like a very nice gun.

I guess we are lucky in the USA in that shooting antique guns does not change them into being classified as modern guns.