As usual, Doc is 100% correct.
The frame and hammer are made so that the nipples of the cylinder are offset from center of where a cartridge hammer would fall. It was purposely made that way so that you cannot just put a cartridge cylinder in it and expect it to work. I've read several articles about this that explained how it would be very difficult to convert it to use a cartridge cylinder. One article was very good in that it showed pictures of how the frame, hammer and cylinder are offset differently so that a center fire cartridge cylinder will not work.
About a year or so ago I bought two Uberti cattleman .45 Colts that are cartridge revolvers for $250.00 each brand new from Ray Tanner of Tanners Gun And Sports Center, in Jamison Pennsylvania. He had ordered a large quantity of them from Uberti in both .357 and .45 Colt. Not sure if he still has them at that price or not, but you could google him up, call and ask.
Like Doc advised, you'd be much better off in the long run getting a cartridge 1873 rather than trying to convert the black powder 1873 percussion models to cartridge use since that would be a very difficult thing to do.
Although they are currently out of stock, (actually listed as "discontinued") Midway did carry the R&D conversion cylinder for the percussion 1873 Uberti cattleman at this link....
http://www.midwayusa.com/product/171...d-6-round-blue Since Midway has discontinued the sale of that conversion cylinder, you could try calling R&D directly to see if they are still available from them.
Here's an interesting article about how a guy had his Uberti 1873 cattleman percussion go full auto on him when he tried to convert it to use cartridges. Does make for an interesting study for anyone who might be interested in making their revolver work like a semi auto though, via primer blowback cocking the hammer. But in this case, it didn't just cock the hammer, it made it go full auto. You'd need some sear catch work to make it semi auto.
http://www.shooterslegacy.net/forum/...hp?f=16&t=2454
.