Pappy, I wish I could be more help on this one, but I honestly am not sure. Being a Continental weapon, it could have been metric. I don't know if the Austrians were using metric measurements at this time or not. The French appear to have been using it, as their bore measurements were often expressed in mm. The Lorenz that I had was an oddball. It was obviously a '54, but was a flintlock! The lockplate was stamped 859 (1859)but appeared to be original flint. Apparently it was originally a Consol-Augustin tubelock which used a primer holder/firing pin mechanism that was held shut with a frizzen spring. The tube primer was inserted into the touchhole. Someone converted it by replacing the cover with a frizzen and the hammer by a flint cock. There was no bolster or nipple. Many were imported to the northern states and had to be converted to our percussion system because it was hard to keep these special primers in stock. Why a Consol lock was on a '54 Lorenz, I can't explain. As for yours, I have read that many Continental guns had to be re-threaded in the bolster to accept U.S threaded nipples, so I wouldn't be surprised if it is metric. Also, if you can, it might be a good idea to remove the breechplug and see if the barrel was equipped with a "tige", which is a stem attached to the breech. It protruded into the barrel for about 1 1/2 inches. A round ball was rammed against this to upset it into the rifling. Some of the rifles were equipped with this. The Austrian muskets were not. This thing can cause problems in cleaning and was objected to for this reason. The original caliber was .54 for the Lorenz, but many were bored to .58 and re-rifled. That fact, and corrosion and wear would explain the 20 gauge bore. I know this is a lengthy answer to a simple question and I appologize for that. Especially when I'm not sure of the answer. I just happened to think of some other points. I do like to talk about CW period guns. We probably should move this topic to another category though if we want to continue it. We've kinda drifted away from the smoothbore aspect of it. I do have more printed info about Austrian guns from this period and if you need anything else I'll keep my eyes open on the percussion forum if that's the best place to do it. Have you got to shoot that fowler yet?