Cerrosafe could be a bad idea. Many of the thin barrel lever gun have slight bore indentations where the dovetails for the sights were cut. Also some bores are tighter at the muzzle than further back, and vice versa. My point is, the cerrosafe could be locked into the barrel and have to be melted out by heating the barrel. Not good. Also, if you cast it around a screw, you risk differential coefficient of expansion of the metals affecting your reading.
Slugging is done with pure lead only. Cast bullet alloys are too springy, and that not only makes them hard to push down the bore, it causes oversize readings.
In general, you can cast bullets of pure lead that are slightly over bore groove diameter and slug with them. If you can still find egg-shapped fishing sinkers that are pure lead in your area, you can use those, but I find they are all harder alloy around here, lead having been phased out. You can buy Hornady makes round swagged lead balls in 0.480" and 0.433" that are pure lead and that should be about right for your guns. Roll them between two flat pieces of metal until they are just a couple or three thousandths over groove diameter and slug with them.
You would ideally slug the muzzle and breech and bore length separately, but that would be a challenge in your rifles. You can make two measurements, though; the muzzle and the bore overall.
To proceed, I would first pop down to Lowe's and buy a brass rod that will slide down both bores. 3/8" looks like it will fit both your guns. Cut a 3" piece that you will use as a punch with a hammer (plastic head is safest) to tap slugs into the muzzle to start them. Get the lead balls. Bigger is fine, but you'll spend more time rolling them or else you can actually leave them round and let the muzzle shave them, but it's harder.
For the overall bore measurement, run a lightly oiled patch down the length of the bore, then wipe the lead with it. Tap it into the muzzle. Once it is completely in, use your brass rod to slowly push it through the bore, feeling for constrictions or rough spots. If you have any significant constrictions, they may have to be removed by firelapping or hand lapping (don't try that on a valuable gun your first time) before you get best accuracy from the gun. Measure the slug across its groove impressions after it comes out. Measure across every groove set to determine roundness or constriction deformation.
For the muzzle, simply tap a slug part way in and pull it back out and measure the impression. If that doesn't work, or you have a lot of muzzle wear, you can cut the brass rod so that when it is back against the bolt face it leaves you with a couple of inches of unoccupied muzzle. Tap the slug all the way into the muzzle, then tip the muzzle down so the rod slides forward, at which point you should be able to push it out with the bolt or your finger on the rod. Again, measure all around. measure the end that went in last and the end that went in first to check for muzzle funneling.
I strongly recommend you not try to measure the slugs with a caliper unless you have a dial type that lets you read between the graduations, a calibration standard like a pin gauge near the bullet diameter, and a history of skill with the tool. It is very easy to miss by a thousandth with a caliper making this kind of measurement. The tool you want is an O.D. thimble micrometer with a vernier scale that lets you resolve tenths of a thousandth. You will need to wipe its spindle nose and anvil and check its zero for the temperature of the room, but it's an inexpensive way to get a pretty precise reading. Once you know what size to make bullets, even without calibration it will let you compare them. This is an instance where an
inexpensive import type will be adequate, because being off 0.0001" or even 0.0002" won't cause a problem. In addition to that,
Harbor Freight has a $20 one with a counter on it for the coarse numbers and Vernier scale for tenths. I haven't tried theirs, but it's probably good enough, too.
As an aside, I have found my lever gun (Marlin 1895) and revolvers all are more accurate with cast bullets 0.002" over groove diameter. Assuming that doesn't make a cartridge too fat for your chamber, it's something for you to try.
