By all means use the 410 cases, not 45 brass, as it gives far too much jump to the rifling with room to almost lay sideways for almost two inches, with powder gasses blowing by all the while.
Since the gun is rifled it will handle solids very well, but I recommend gas checked bullets not plain base, and experiment a bit to find the largest diameter that will chamber easily, rather than just fit the rifled bore. This large diameter will hold the bullet straight while it is moving from it's seated position to the rifling, and permit good accuracy. Bullet length should not be more than a normal foster slug, and don't expect to get all the way to 1700 fps with shotgun hulls and they won't stand the pressures that brass cases will.
The information I need to have sent with an order is groove diameter of the barrel and maximum diameter the cases will hold and chamber easily with. I will design a bullet which fits the case, yet doesn't require excessive sizing for it's full length when it gets to the rifling. It will use 45 caliber gas checks, which may or may not crimp on if the bullets are sized, and it may not be possible to make the most accurate bullet and have it capable of being sized and lubed in a normal sizer. It may need to be finger lubed, which is no problem for a gun which won't be shot by the thousands of rounds.
When Contender came out with their 45/410 barrels I developed a bullet to fit the 45 brass which worked quite well, but have forgotten to many of the details to recommend the final design. I do remember that it had a large body to fill the chamber quite closely, with a heel to go into the case a short distence. It worked fairly well but took quite a bit of experimenting to get a desine that worked. Be experimenting I mean several experimental molds sent one at a time to the gun writer, tested, results returned to me, then another design cut, etc, etc. No one ever bought a mold, after all our work and getting the magazine article published, and that was probably at least 15 years ago. -- The last and most important bit of advise on this matter. Though I would dearly appreciate a mold order, I doubt that you will get better performance than you can with standard factory slug ammo, and the price of a mold will probably purchase a lifetime supply of loaded ammo that works without any hassle. Consider that before you purchase a mold and venture into what may become a long extensive experimenting process without complete satisfaction when you are done. By complete satisfaction I mean being able to produce ammo with performance that exceeds that of factory loaded slugs. I think I've probably met close to a hundred 410 slug shooters who use the guns primarily, or only for deer and believe they have the ultimate 'deer rifle'. 410 slugs are unbelievably whimpy so far as ballistics go, but deer are easy to hit with them at normal woods ranges, and they tend to die quite promptly when hit, with minimal meat loss. I've learned over the years that the shooters impression of power is far more important than actual power if his impression is gained by field experience. -- For example, I knew a young man when him and I were both in our youth who believed that his 22 auto rifle shooting the 22 LR was as powerful as a 30-30. He definitely learned better after getting a more powerful gun, but he got that crazy notion because he had killed probably 30 deer with it, all poached, for a large needy family, with an ill father who couldn't provide for them, and was too 'proud' to take welfare. I'm pretty certain about all this because I married this boys sister 47 years ago.
For those interested in the type of performance which such a gun would be expected to produce for deer etc, don't purchase the combo barrel. I recommend a 44 magnum rather than the 45 Colt case, as the throatings seem to be better suited on the slugs I've measured.
