Chance I have not researched options in sabots for the 45. Shooting a .357 Hornady 180 XTP from a Muzzleloader would not be bad at the 357 Maximum velocity the bullet is designed for. I would think it would be flatter shooting than a 44 bullet, and quite possibly have more energy at longer range due to its better ballistics.
I was working with cast bullets in my 357 Maximum a little a couple years ago. I believe my mold is a NOE 360180 Gas Check. I was getting a little leading in my Contender barrel that I believe is .358. I ordered different bullet sizer dies, but never got back to working with it. (To Many Toys, To Little Time) That bullet when shot from a Muzzleloader barrel using a SABOT could be ran at any hardness, and at any speed you could wind it up to with no leading issues to worry about. The only thing needing a little thought is just how soft to make the bullet with leading not a concern at the velocity generated.
I was originally thinking that Hornady 180 XTP would be the bullet of choice in the 45 barrel. Another one worth a look is the FURY 180. It has a very similar profile to the now discontinued Hornady 180 SSP, and per my testing expands about identically to the 180 SSP when shot from my 35 Bullberry. This bullet is way more aerodynamic than the XTP. It may be a little to heavily constructed for 357 Maximum velocity however. I would want to test it before hunting with it.
After more thinking the MGM 45 barrel with 357 Sabots might be very hard to beat with a good 180 cast bullet. If I were to get the MGM 45 barrel it would probably never know any bullet existed except the NOE 360180-GC. I am confident this bullet driven at Full House 357 Maximum Velocity will get the job done.
I worked with all kinds of powders and pellets in my Optima.
I was not impressed with any of the Pellets. Blackhorn 209 was by far the cleanest loose powder I tested. HOWEVER I did not get around to working with Black MZ. From what I read Black MZ is supposed to be about as clean as Blackhorn, and can be bought much cheaper with a little shopping around. I would probably work with Blackhorn 209 or Black MZ powders in the 45 if it were me working up loads for it.
Keep us posted. This should be a great project barrel.
See You At The Range