Don, you are right about alloy. You want to use alloy 25/1 or softer. I prefer 30/1. The first time I tried to muzzle load it was with some Bullets made up with linotype and pure lead, I have no idea how hard. But the first time I tried to drive a bullet thru the false muzzle I went into shock. Hurts when you give a bullet starter a whack and the bullet just barely goes into the false muzzle. I cast some 30/1 alloy bullets and all went well. Now as to size, my Pope slugs at .380 to .381 depending on where you are in the barrel. The original mold that came with the outfit casts .382 so it is groove diameter or a little bigger at the base, I have 32-40 pope molds that cast .326 to .327 at the very rear band. They shoot super breach seated with smokeless, have not tried them muzzle loaded. The scrapes the barrel clean theory is real and it does work. I have shot my 38-55 many times with black both straight and duplex loaded and after you have pushed a bullet all the way down to the chamber if you give it a little snap at the end it will fall out thru the throat and the bore will be claen and bright and shiny. This also by the way leaves a little film of clean bullet lube in the barrel in the process.
Now as to the process. I have done this quite a bit. I have shot a couple of 100 and 50 shot matches and doubt I will ever do it again. maybe a ten shot match where straight black is the rule. I found that my rifle shot super muzzle loaded. BUT! it is a lot of work. let me describe the routine.
First you have to shoot a fouler just to limber up the barrel and the lube in the barrel.
then you stand the rifle up, wipe the muzzle clean, mount the false muzzle. start a bullet into the bullet starter, mount the bullet starter carefully onto the false muzzle and drive the bullet into the barrel. It goes down about 3 inches. then you remove the bullet starter, and then using the seating rod thru the false muzzle seat the bullet carefully down to the chamber/throat. now remove the seating rod and false muzzle and park them securely. now you can bring the rifle up to the horizontal and look into the chamber to see the base of the bullet. Some people have experienced the seating rod "sucking" the bullet back up the bore although I have never experienced this. A shot fired with the bullet some where in the barrel other than at the breech is asking for a "Ring or Donut" in your barrel. You are now ready to load in the case with the powder charge, I use a cork wad in the mouth to keep the powder contained. You may now bring the rifle to battery and aim and fire. Now the whole process starts all over again. By the end of the day you have hoisted this 14 or 15 lb rifle up and down and done all kinds of inbetween work and you are exhausted. NOT FUN!
I shoot now with the same bullet breech seated, mostly with smokeless,
sometimes with black duplex loaded when allowed. and I do much better. That is to mean I do much better. I will say that when I muzzle loaded using either straight black or duplex the rifle shot superbly. All the Pope molds I have had the opportunity to examine and or cast from are tapered, usually from bore size or even slightly smaller at the front to over groove size or even bigger at the base band. With good lube they shoot well in good rifles. I own a couple of original Winchester 85 s in 32-40 and that big base band Pope bullet seats easily in both of them and they do not appear to have ever been rethroated since leaving the factory. Well enough for now. HTH, Regards, FITZ