Leading that starts at the breech end is a definite indication of undersized bullets. Shooting cast bullets is not nearly as simple as casting a tumble-lube bullet from wheel weight metal, which can vary considerably, rolling it in LLA and loading.
First, the throats need to be larger than groove diameter. Slug the barrel and see if the slug will pass freely, or with very slight resistance, through the throats. If it does, good. If it doesn't the throats need to be enlarged until the bore slug will pass easily.
Once a bore slug will pass through the throats then measure the throats by slugging them. Size your bullets so they are throat diameter to .002 larger. Notice that you don't have to know the groove diameter!
Slugging the bore. Drive a slug into the muzzle and all the way through the bore and out the breech. If it passes through the frame area without extra resistance that is ideal. If you meet resistance in this area your barrel has what is called a frame or thread choke, the bore diameter through the frame is smaller than the front of the barrel. This choke needs to be removed by lapping. So-called "Fire lapping" will do the job. Fire Lapping kits are available from many sources, simply Google it. They come with instructions.
If your barrel has a frame choke, checking and fitting the cylinder throats must be done with a bore slug that has only been into the muzzle of the barrel, not pushed through the frame choke area.
But, before you do anything else, it will make life easier to know the groove diameter at the muzzle since that is the point to which fire lapping will be designed to enlarge the rest of the barrel to. If your barrel is larger than .432-.433 it won't be worth the work and cost for a custom bullet mould and sizing dies to make bullets to fit it properly. Believe me, BTDT, and I have been doing this a lot of years.
If you want a bore slug measured PM me and we can work something out.