Bore is smooth, great to load. Box is marked 60, it is probably bigger than .600. Didn't measure, set came with a .600 mould tho I don't think it was ever used. The 45 marked box, came w a .451 mould and the first loads at that size were pretty good. Will let u know what it finally settles on
It is great that it is smooth.
The exact size is necessary to make an infomred decision on what size to even start with.
As I noted above, the fact the .600" ball caused burning shows it must be smaller than the bore.
I'm sure this is all really exciting for you, being new to muzzleloading as it sounds you are, but using undersized balls in a bench-rest barrel is a waste of time and powder, not to mention being most frustrating as well. I would expact that using a ball that is Bore size, is the first step, then going larger in .005" increments from there is the best method.
Measuring the bore and groove diameter is a VERY easy thing to do - simple dial calipers will work, you will also need an oversized PURE slug of lead to pound into the bore, then extract and measure properly. You might also be able to measure the bore and groove without a slug of lead, but the measurement wil not be as perfect as if the slug is used.
To extract the pounded in slug of PURE lead, drill an undersized, hole, screw in a wood screw, then grab the wood screw with pliers, then use a hammer to tap on the handles to extract the slug. Measure to get exact bore and groove measurements. Screwing a large diameter, size 12, for example wood screw into the slug will expand the slug perfectly to match the bore (this is in addition to having to pound it into the bore to start). You want a perfect reversed impression of what the bore looks like to get an exact measurement.