Out with the .45 Trapper

Over the years a number of leverguns in .45 Colt have exhibited excessive degrees of case expansion. Similarly, no small number of sixguns have also fallen victim to various chamber gremlins made apparent by the spent brass.
The reason for this condition is poorly understood, however in a nutshell, it has to do with the chamber taper. The .45 Colt is a straight walled case in a tapered chamber. This taper worked great in black powder sixguns, because it enabled extensive firing despite the notorious fouling associated with b.p., and allowed for the rather weak copper cases of the time to fully eject. Tapered chambers also worked because the original bullet design was in essence sharply conical. The explosive force of b.p. bumped up the bullet to seal the barrel/cylinder gap and fully engage the rifling.
Unfortunately, smokeless powder and modern bullet profiles create pressures and dimensions that aren't served by the traditional chamber dimensions, and when subject to the inevitable variances of mass production, can result in excessive dimensions that really challenge the brass.
There are some techniques in reloading that can mitigate this, depending upon some variables, but with factory ammunition the best solution is to fire standard pressure loads with new brass.

Thanks for the detailed explanation Mac. I sort of knew the issue but I thought the case was tapered and the chamber bored straight. I'll defer to the experts on the matter and take your word as correct over what I assumed was going on. 

Probably the fastest way to discover if a case is straight is to see if there are carbide dies available for it. If the answer is a yes, then you have a straight walled case, because carbide dies can only work correctly on straight walled brass. 
I should mention that in the real world nothing is perfect, thus even straight walls can exhibit a minimal taper, but that small taper can still work with the carbide ring, which is not the situation for a truly tapered case.
I've had the pleasure to be shown some drawings of the .45 Colt chamber, and the chamber when loaded. Helped to make it all clear.

ALMOST TRUE. THE 9MM IS TAPERED SLIGHTLY AND CARBIDE DIES ARE AVAILABLE

Off hand, I don't have the information on the degree of taper most modern carbide rings will size. Like I said, carbide will size minimal taper. Anyhow, just like all rules of thumb, it's not meant to be definitive, it's meant to be a guide. I hope you'll agree that for any serious shooting endeavor, it's up to the man-or woman-to acquire the complete information for the task. However, in this case I do stand by what I wrote.

The carbide dies I use/ commonly available "Cheat" on slightly tapered cartridges, don't really create a  taper, just size to the smallest diameter 3/4 or so of the length. Most steel dies do have the taper in them, that is likely why rifle carbide dies are crazy expensive, instead of a ring of carbide in the base the whole bore of the die is carbide. (I think...)

Reloading dies don't resize the cases to exact replicas of the design, they get things close enough to function most of the time. There are several examples of this that come immediately to mind. Always has been a tension between engineering and bean counting, maybe more of an unholy alliance if we're truthful. Lots of examples here as well, and that's a big factor in this equation.
Anyhow, I think the result is as you've described, a sort of frankensizing of brass that works if the taper is minimal.