When I started with black powder, both cartridge and muzzle loader I kept hearing about twist rate and what to shoot with what twist rate? My mentor told me he made a measuring rod used just for that and here is what he did. Got a one piece of aluminum rod (long enough for any barrel you have), threaded the end so a patch holder could be put on it. I used a slight amount of epoxy to prevent coming loose. Since most barrels were 45 and 50 caliber one rod was all that was needed. There were several pieces of rubber tube on the rod so the aluminum would not run off in the barrel. The rod was measured from the tip of the patch holder in 1 inch increments and marked with a line straight down the rod and marks at each inch. Non removable ink or marker was used.
The first time I used it was to measure a Hawken kit I had assembled and did like this. Run the rod to the bottom of the barrel and put a piece of blue painter's tape at the muzzle end. Put several patches or enough so the rod was a tad hard to push in. Start at the bottom and pull the rod out and watch to see when the long line on the side of the rod makes one turn and count the number of inches it had to travel. If the barrel is a slow twist then measure a one half turn and so on. I record the numbers in my log book as many rifles have no markings.
I did not mention what to do if the measurement looks "strange". In this case measure at the breech, helps if breech plug is out and then measure from the muzzle to see what the readings look like. I bought an unmarked 45 caliber barrel and the twist rate appeared "strange". So I measured from both ends as the breech plug had not been installed for good. At the breech it was 1 in 20 and muzzle it was 1 in 15! The barrel was about 30 years old with no markings at all, but the rifling was like that of Pope and one or two other more modern barrel makers.
John