Rusty: If you have access to a caliper, you can measure the inside of your barrel at the muzzle and get a fairly good reading on its bore diameter. ( land to land) The other way is to hammer an oversized soft piece of lead or ball down the barrel and then pull it out, and measure the lands and groove left on it. I would not invite anyone to dry ball a barrel, but you can put 5 grains of 4Fg powder down behind it, seat the slug on the powder and shoot it out into a stack of newspapers and recover it pretty much intact. I have slugged the barrel on modern rifles, where I have access to the breech, but I have also waited until I did dry ball my ML and then used a micrometer to measure the slug one I pulled it from the barrel.
Some barrels have odd numbers of lands and grooves, and then you need a barrel mike to measure them. There is a gentleman in Virginia who offers this service free, All you have to do is send him a slug taken from your barrel for him to measure. I would take a .580 diameter round ball, give it a tap to fatten it in one plane, and then hammer it into the muzzle of the gun barrel. While it is in sight of the muzzle, I would screw in the ball puller jag I have and then pull it back out, to get my measurements. Put a witness mark on the slug, and put it aside. If you ever need to polish the barrel, that slug will work wonders with some valve grinding compound, or JB Bore Cleaner. All you would have to do is wind it back on your jag, line up the witness mark so you set it into the barrel so the lands and grooves line up, put your valve compound, or JB bore cleaner on the slug, and run the slug down the barrel, and then back and forth to work out a rough spot.