I worked on this today and have made some good progress.
The rounds I made earlier were much too short. To determine optimum length using the Lyman 515139 bullet, I started out by determining what the maximum length cartridge the action would handle with this bullet. I started out with an OAL of 1.700". I loaded one round and tried to cycle it. That was too long; it would not completely exit the magazine. So I started shortening the OAL until one round would clear the magazine and cycle. That ended up being 1.680" OAL. So then I loaded a second round to the same length and loaded them both. The first one cycled in, but the second one's nose got caught and cocked the bullet in the case a little. The problem was that the Starline brass is not long enough to support the bullet at this OAL. I had previously ordered 50 Starline .50-70 cases, in case I needed them for the Spencer. I wanted the case to cover the lube grooves, driving bands and fully support the bullet. So measurement of the 1.680" OAL cartridge indicated that I needed a case length of 1.356". So I cut down and trimmed one .50-70 case to this length, reduced the rim diameter to .645" and beveled the rear edge of the rim. Ran it through the .56-50 sizer, then tried it in the chamber - it fit nicely, but the rim was too thick and the block would not close completely - Grrrr.
I measured the Starline rim thickness. It varied from .057” to .059”, the .50-70 rim was .065”. So I thinned the rim (from the front edge) to .058” and tried it again in the rifle. It chambered fine and the block closed snugly. So at this point I made up a dummy, and loaded it and the Starline cased cartridge of same length, the longer cased cartridge being second in line. I tried to cycle them and they both worked well, although the second bullet's nose had a little divot from the UBB. I was pleased at this point. So I made another cut down .50-70 case and tried two dummy rounds , both cycled well, but still getting a little divot on the second round’s nose. I went on to make 3 more cut down .50-70 cases and dummy rounds. I loaded up all five and 1,2,3 and 5 cycled well, but it got hung up on #4 - Hmmm. Then I remember reading on the forum that adding a little grease to the bullet noses helps a lot. So I tried it and SHAZAAM! – all 5 rounds cycled, chambered and ejected nicely.
I think I’ve got a workable cartridge now. I’ll make up 5 more cut down .50-70 cases this week, then load up all 10 and go to the range and give them a try. My bullet alloy is wheel weights with some tin solder added, somewhere around 25:1. I think a harder alloy will help the nose hold up better to the UBB’s bite. I’m not sure if I should do any more work on the UBB’s leading edge or leave it as is. Overall it’s been a productive day. I still have some work to do with it, but I’m pretty close.
After this, I’ll start all over again using the Rapine 350T bullet….