I haven't spent a whole lot of time looking into it, but.............
The 400 CorBon is a lot like the 357 Sig.

It's a necked down case to take the next smaller bullet size. The 357 Sig is a 40 caliber case, necked down to take a 9mm bullet. And, the 400 CorBon is a 45 caliber case, necked down to take a .40 bullet.

Unfortunately, you cannot use a .40 S&W case, and just neck it down to make a 357 Sig. And, even dealing with a 10mm case doesn't get the final dimensions right.

The same is likely true for the 400 CorBon. I doubt that the reduced neck on a 45 ACP case will result in a true dimensioned 400 CorBon, hence your issues with feeding. And, the grip length for the bullet would be wrong, as it was for the attempt to convert .40 S&W to 357 Sig.

My edition of Book of Conversions predates the development of the 400 CorBon. However, Wiki suggests that some trimming is required: https://en.wikipedia.org/wiki/.400_Cor-Bon

I love my Sig P229 in 357 Sig, and have enough brass to serve me a lifetime. I don't see much 400 Cor-Bon brass out there. So, good luck with your project.