rberlalatto, the 38-40 is my favorite cartridge for Cowboy shoots and I have always loaded it with blackpowder. I have experienced this problem you mention mostly in Uberti single actions, to a smaller degree in Colt single actions and not at all in Uberti lever actions.
The problem, as I understand it, is that most factory sizing dies do not re-size the fired case back to its original dimensions. Specifically, the shoulder of the case, upon firing, is expanded forward and most sizing dies do not push it back far enough. I had this rude surprise at a match after acquiring a pair of Uberti Cattlemen and using previously reloaded ammo that worked fine in my Uberti 1866s' and "73. At the match, most of this ammo would not chamber in the Uberti pistols but continued to function fine in the rifles. My fault for not checking this in advance and lesson learned! Anyway, about this time(mid 1990's) I read an article by Dave Scovill in Legacy Of Leverguns, first edition, describing this unusuall situation with 38-40 fired brass and sizing dies. Dave's solution was to contact RCBS and have them custom make a sizing die that pushed the fired case shoulder back to its original, unfired position. Dave also provided a part number. I called RCBS and ordered one myself ("38-40 SCOVILL FL SIZER, PN55010 ). I have been using this sizer die now for almost 15 years and never once had that same problem occur again.
You would think that this much resizing would overwork the brass, but I can't remember the last time I lost a 38-40 case due to split neck or shoulder. I have about 2,000 38-40 cases, mostly Starline but some Winchester, that I bought those 15 years ago and of course I shoot a number of other calibers so that those 38-40's sometimes get a rest. I have often wondered about the disparity of the 38-40 chamber dimentions among manufacturers, especially Uberti. After all, they(Uberti) chamber this caliber in both their rifles and pistols, so why the problem? Doesn't matter, the 38-40 is still my favorite cartridge for the Cowboy matches as it strikes a nice balance between large caliber, case capacity and moderate recoil.
By the way, by using the Scovill die which resizes the case to original dimensions, I have been able to utilize a grease cookie which most people will tell you is not possible with this cartridge. With the now long case neck, the same as new, unfired cases, the base of the bullet ( I normally use Lyman # 401043 ) will not protrude below the case neck which leaves a little room for a card wad and the grease cookie. I don't always load this way, but when I shoot a two day State Match, usually Purgatory in the Pines in NJ, I can shoot all ten stages over the two days, again all this with blackpowder, and I never touch the guns as far as cleaning. This has worked for me for 15 years now so I guess I'll continue.
Sorry to ramble on so long, but when someone mentions the 38-40, it gets my blood going a little bit. Good Luck, Navy Six