Xtriggerman,
I don’t have a direct answer to your touch hole size question, but have some related observations. In my den now are two contemporary Ferguson ordnance rifles, one by Narragansett using Rifle Shoppe parts based on castings from the Milwaukee (Nunnemacher Collection) Fergusson ordnance rifle, and the other by Ernie Cowan using all parts of his manufacture, directly from measurements and observations he made from the Morristown Ferguson ordnance rifle that he disassembled.
The touch hole on the Narragansett rifle admits a number drill bit measuring .068”, and no larger. That vent is located about 1/4” BEHIND the front of the screw plug, and the plug has been severely notched to allow the vent to communicate with the chamber. The vent actually enters the barrel within the female threads for the screw plug, and will be subject to having fouling ground into the touch hole from the dirty threads.
That seems to be the same problem you are solving with your angled vent liner. (Smartdog overcame this problem by stocking his Rifle Shoppe barreled action from his own plank of walnut and placing his self-made lock where he, and Patrick Ferguson, wanted it!)
Cowan’s touch hole admits a drill bit of .079” and enters the chamber directly, just like the photo of the Milwaukee barrel shown on page 57 on Bailey’s book “British Military Flintlock Rifles”. This arrangement seems to mirror Ferguson’s original intent, and rightly so.
I have photos of both vents inside the chamber, and both plugs. If anybody needs to see them, I can make my first attempts at posting photos. Cable, of this site, owns the Narragansett rifle in my den, and said he would help with the posting of photos if and when needed.
These are interesting issues with a fascinating rifle promoted by a brilliant British officer who made this technology and rifle a part of American history. The British hoped that the Pattern ’76 rifle (1000 made) and the Ferguson (100 made) would counter and neutralize the American long rifle, which was giving such heartburn to British Officers coming to America to face American accuracy and ungentlemanly conduct. If Patrick Ferguson, gentleman that he was, had not passed up his shot at General Washington at Brandywine when he had him in his Ferguson rifle sights, we might all be speaking BRITISH now, God forbid!
(Moderators, please allow this thread to persist, since the development and implementation of the Ferguson rifle is intimately tied to the ALR!
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Bill Paton