Jim, to answer some of your questions:
I don't know if the .410 shotgun brass is heavy enough to use in the .450 Mongo configuration (although it very well may be.) That is why I use the .444 brass, which is designed to hold more pressure than I'm wanting to shoot in the .450 Mongo. And, forming the .450 Mongo brass is quite cheap and easy to do from the .444.
My first tests of the .410 brass with a full load of No 8 shot worked very well. I used the Lil Gun .410 formula of 13 grains with a Remington SP410 plastic wad column holding a half ounce of shot sparked with a large pistol primer, topped with hot candle wax to hold it all in, and the 45/410 barrel performed beautifully. This combo is enough to take out any snake known to man. At close snake-shooting range of 10 yards or less, the choke is not needed. When I'm in the field, I usually keep the choke in my pocket and out of the barrel, thus giving me instant choice of shot or solid. Usually, I need the shot only for snakes, which can be expected at any time during the hot months of the year. Of course, for targets at farther than 10 yards, the choke should be used as it pulls the shot in much tighter, which is good for maybe 25 or 30 yards or so as are all .410s. Since the consequences of shooting a solid round through the choke can be a destroyed choke, I never have the choke in unless I am expressly hunting game with shot.
For the .450 Mongo configuration, I fireform .444 brass using a 240-grain .44 lead slug which hardly touches the lands as it is fired in the big barrel, but it still flies surprisingly straight. I have used the same powder charge as the .410 Lil Gun formula noted above, 13 grains. I initially used a paper towel wad to fill up some of the air space between the powder and the bullet in the large case, but now I use Dacron, which is usually recommended for such purpose. There is a lot of space for more powder, and I believe that loads that equate to 45/70 performance might be achieved. However, since this is a wildcat, one must experiment carefully. You are drawing the map as you go.
Relative to your question about sizing dies, I use my .45 Colt dies to resize the top of the .450 Mongo case to hold the .45 caliber bullets. Thus far, I have not had to resize the base. If I ever do, I may sacrifice a .45 Colt die by cutting off the top, thus allowing the full .450 Mongo case to be resized to .45 Colt dimensions. An alternative would be to use a .444 die, but, this would size the cases down farther than needed and would necessitate using the .44 bullet for fireforming again or using a graduated punch to open up the mouth to .45. By using a .444 die, you are effectively repeating the fireforming sequence again, which can't be easy on the case. That is why I will consider altering the .45 Colt die if and when the need arises.
In my opinion, the .45/.410 barrel in the Contender is perhaps the most versatile factory weapon ever designed. With shot and solids in your pocket, you are good to go for just about anything in the world short of some really heavy and dangerous animals, and even some of those have been killed with less. I believe it may be the ultimate survival weapon of all times. I know it would be my first choice.
