Howdy Again
Here are a couple of photos I found on my hard drive. For those who are not familiar with the 45 Colt and the 44-40, here is a photo. Left to right in the photo the cartridges are 44 Special, 45 Colt, and 44-40. You have to look very carefully to actually see the taper of the 44-40, it is barely a taper at all.
Here is a photo of one of the 44-40 cases I fired in the 45 Colt revolver. It is the one in the center. The case on the left is a 45 Colt, the case on the right is a 44-40, both having been fired in guns they were chambered for. The 44-40 in the center has bulged out at the case mouth to completely fill the 45 Colt chamber. But further down, the case has maintained its original dimensions. As you can see, there is no splitting at all of the 'expanded' 44-40 case. None of the five rounds I fired exhibited any splitting. But the mouth has expanded enough that it will never be sized back down by a 44-40 sizing die. The reason that only the case mouth bulged, and not the entire case is because 44-40 brass is extremely thin at the mouth. Only around .007 thick, vs around .012 thick for a 45 Colt. Further down, the case wall thickens, and that is the part that has maintained its original shape. But the very thin brass at the case mouth expanded to completely fill the 45 Colt chamber.
Test gun was a 2nd Generation Colt Single Action Army, with a 7 1/2" barrel, made in 1973. Chambered for 45 Colt. The load in the 44-40 round was 2.2CC (around 34 grains) of Schuetzen FFg Black Powder, under a 200 grain pure lead .428 diameter Mav-Dutchman Big Lube bullet lubed with SPG.
But knowing what I know about the 44-40 round, I would not hesitate to try one of my Smokeless rounds, with 7.5 grains of Unique under a 200 grain RNFP bullet either.
As far as accuracy is concerned, as I said, I was able to put 5 rounds onto a 16" square steel plate offhand at about 10 yards. I have no idea what the group size was. The bullets may even have been keyholing for all I know. But they still all hit the plate.
Why did I do it? Because I wanted to see for myself. I had always heard that trying this trick, the bullets would be so inaccurate that the shooter would not be able to hit anything. From the standpoint of Cowboy Action Shooting, accuracy was good enough that if I had run out of 45 Colt ammo that day, I could have continued the match with 44-40 without so much loss of accuracy that I would be missing targets. Since all my cowboy rifles are chambered for 44-40, I always have some of both rounds with me at a match.
No, I would not try to put a 45 Colt round into one of my 44-40 rifles. That is a different story. That is a guaranteed jam, the round would feed through the magazine, but would jam when the bolt tried to shove it into the chamber.