You can slug a bore with a slightly flattened pure lead ball. You can also round out a lead fishing weight. Anything that is slightly over groove diameter can be used to slug a barrel. I have also slugged barrels with standard hard cast bullets with good results.
The trick is how many grooves are in the in the bore.
My Trapdoor left the factory in 1883. It only has 3 grooves. I cannot speak for all Trapdoors, but that is the story with mine.
It is easy to get a measurement on a slug shoved through a bore with an even number of grooves, you measure across the slug between two high points and you are measuring groove diameter. With a bore that has an uneven number of grooves, if you measure across the slug with calipers, you will be measuring from groove to land. Then you have to add the depth of one groove to get the real number. This is pretty tricky, I have never been really good at it. It is tough to balance the depth measuring end of the calipers on the edge of the groove and get an accurate measurement.
I probably slugged this barrel with these bullets years ago, but I don't really remember, so I just redid it. These are the .458 diameter bullets that I always use in my Trapdoor and my Sharps. I measured one of the bullets and actual diameter is running between .4580 and .4585. After slugging it was about .455 across from groove to land, add about .001 for the depth of the groove. So that means my Trapdoor probably has a groove diameter of about .456. Give or take something for the wobble of my caliper.
The grooves are very shallow, but I did get good drag marks across the high spots, corresponding to the grooves. That means my slug pretty well filled the grooves. I have always had good results with these bullets in my Trapdoor and my Sharps. Dunno if I could do better, but at least the rifling gives them a good spin and they don't keyhole. The bore of my old Trapdoor is actually nice and shiny without any pitting.
P.S. Yes, there are techniques for measuring an odd number of grooves using a V block, but I have never tried it.