M40-11, a couple of questions that I hope I can maybe help you out with an answer or two:
Evidently you didn't get a set of dies with the rifle, correct? If not, I know Lee made, hopefully still makes, .43 Spanish dies that are reasonably priced. Any other .43 Spanish dies may be hard to find, and no doubt will be fairly pricey. This is just from what I experienced about a year and a half ago.
Am I right that the cartridge case you show with the chamber cast is an actual fire-formed case from the rifle? The reason I ask this is because I have an original #1 sporting Rem Roller in 44/77 cal., and my fire-formed cases look very much like yours. My new, unfired Jamison cases that I got with the rifle look quite different at the case body transition to the neck than a fire-formed one does.
There's a lot of similarities with those calibers, i.e., the .44/77 and the .43 Spanish. I've never heard of the "Reformado" designation before, though. You will need to slug the bore to find your true bore diameter if you want to be serious about accuracy, along with the proper mould to fit it to the groove depth (and that's nothing more than my opinion). I think there may be a few that would agree on that... I slugged my roller from the breech into the rifling a few inches, and then backed the slug out and measured it. Then I did the same from the muzzle, backed the slug out and measured. I found it hard to do a good, confident measurement with my calipers, because the grooves in my rifle don't line up across from each other to get a proper measurement. I ended up finding and using the right pin gauge to slip through the bore and found my bore diameter was (is) right at .437." Mike Venturino, in his book, "Shooting Buffalo Rifles of the Old West" says bore size in these old rifles of .44/77 had quite a variance in bore size (what I had to deal with) and the .43 Spanish had a bore size of .439." The "Reformado" bore, I guess, may be different. Again, slug the bore to really determine what you have (IMO). I was fortunate to get a set of 43 Mauser dies with my rifle, but I also have two sets of .43 Spanish dies that I use together to come up with a good, accurate handload for my .44 to hunt with. I also got a really nice Steve Brooks mould with the gun that when I size the bullets to fit my rifle proper, I've got a cool, old original .44 caliber rifle from the "Big Shaggy" hunt era from back in the day. My rifle, as near as can be determined is right around 1880 as to date of mfg. If you need a custom mould, Steve Brooks can make most anytbing you want. A great guy to visit with, besides.
Another question:
Did the fellow you got the rifle from reload those "poison bullets" (as you call them) and you have 50 of them? I understand they may be brass jacketed or some other mystery alloy (?). Also, what's the R.O.T. on your roller? That could very well determine what bullet weight you'd want to shoot accurately in your reloads. I use 1.5 fg Swiss in my reloads, and use the 77 grain charge. That charge brings the powder right up into the neck, and with very little compression, the bullet seats exactly right for my bore C.O.A.L.
FWIW: Consider trying the pin gauge measurement application. This worked well for me to determine my right bore size. If you know fairly closely your bore diameter, but are fairly unsure to your measurement, order a couple of pin gauges a couple of .000's above and below your findings. Find the one that JUST slips through your bore and keep that one. You can send the other back for a refund (as I did), and Bob's your uncle.
I dillyed and dallyed around to find the right combination of die parts, bullet sizers and the pertinent measurements to come up with a great hunting load for my old roller. It was a fun project to make a true custom load to get that old gun to shoot, and I killed a nice doe whitetail with it last year, a 75 yd. shot through the boiler room with open sights. Quite a satisfying result when you've done what it's taken to produce an accurate piece of ammo for an old original rifle.
You might consider a copy of Venturin's book, "Shooting Buffalo Rifles of the Old West," too. Lots of good stuff in there. Also, that's already been mentioned, Buffalo Arms is an outstanding source for supplies and first rate info of most anything regarding B.P.C.R. I believe they make custom moulds, too. Call them, ask them, and have a good visit!
Wish you all the best in finding a proper load for your roller, and if it were me, I wouldn't shoot those "poison" bullets. Figure out the right cast bullet, lubed properly, and seated over a proper charge of b.p. (just my opinion, and nothing more...). Have fun with a great shooting project!