I used 19.0 grs. of 4198 with an RCBS CSA 300 mold.

I think I might try loading smokeless in this rifle.  I am trying to get a handle on the original black powder ballistics were for the cartridge.  This time Wiki is no help. Doing a little searching and I found one reference to an original factory load using a 265 grain bullet at 1400 fps.
   The vintage Winchester mold I have with its handle stamped 40-82 produces just that very bullet a 265gn fn-pb.
   The only comparable cartridge I can think of for load data is the 401 WSL at 1.5" case length. The 401 works at a way higher pressure than I would ever want to put through a BP roller so no real help there.  No doubt I will figure something out but I would like to be sure that 1400 fps with a 265gn bullet what I should be looking to duplicate with a smokeless load.  Was that the original BP velocity and bullet weight ?

I tried 21 gn of Accurate LT-30 and velocities were hovering around 1300 fps. I used pistol primers as an earlier pressure indicator and they showed no sign of high enough pressure to flatten the primers.  The edges were still rounded . Even with the large diameter firing pin no primer cup material was cratering around the firing pin. I didn't shoot groups but at 100 yards an 18" steel gong was no challenge. 
  I like the load. Its easy on the rifle and the brass and about comparable to a 44 mag load fired from a revolver.

Late yesterday too late to shoot I cast some bullets from an old Lyman mold # 412263 . Its a round nose plain base bullet running about 300 grain in a soft alloy.
  Its kind of an odd two diameter design having two wide drive bands at the base with one wide lube groove between those then forward it drops nearly .002" diameter and has another two wide lube grooves with one very narrow drive band between.
With this bullet with just the bottom lube groove filled it can be seated long with just enough bullet in the case to cover the lube groove.  I found that with this bullet and shallow seating this rifle will then chamber a cartridge with the bullet sized to .410"
   I have a few so loaded to try today. I reduced the 21gn LT-30 I used for the 265gn bullet by one grain to compensate for the increased bullet weight. If the rifle likes this bullet it should be about perfect for general use. I do like the old Winchester 265gn bullet for GP use. 
  I was reading something about a model of rifle called the hunters rifle originally offered in 40-50 SS. It was a shorter lighter weight model for deer hunting. That describes my rifle pretty well. I see it as not a long range rifle but as a short range hunting rifle having all the capability of a 44 magnum revolver but much easier to place shots precisely. 
  I think I might try it for eliminating some gophers on our property. They are driving our dogs nuts and the dogs are digging tunnels to China.

As luck and work would have it I finally hit on a wonderfully accurate load for this 40-50.  I was getting decent accuracy with the AAC LT-30 powder but not as good as I hoped for.
   I have with other BP cartridges gotten excellent ballistic uniformity with a duplex smokeless load using two smokeless powders stacked.  The kicker is usually something in burn rate from magnum pistol powders to small rifle powders say between 2400 and 3031 on the burn rate chart. The main charge is usually one of the very slow burning magnum rifle/50bmg powders
   For this load I used .3CC (Lee dipper) of Accurate LT-30 under 35.5gn H-870 with a CCI large pistol magnum primer and the Lyman 300gn rn sized .408" cast in a BHN-8 alloy and lubed with our NASA lube.  The results are everything I could hope for in accuracy and ballistic uniformity.  First I fired a three shot 100 yard group and they stayed inside 2". The next three shot group was fired through the Ohler sky screens and also stayed inside 2".  Now the really incredible part the ballistic uniformity.  The first shot registered 1358 fps which is exactly the velocity range I wanted.  The second shot also registered 1358 fps.  I thought maybe my battery is low because I thought it missed the second shot. Going through the summary it said there were two shots and the es was zero, the sd ws zero, and the average was 1358. So the two shots were on there so I fired a third shot that registered 1346 fps.  That is a 12 fps es which I believe is quite good.  I always hope for an es number that starts with a one.  That is going to be hard to beat.  

I think so too but I am not ready to quit yet. This rifle seems to have a fairly fast twist so it will be interesting to see how far I can push the bullet weight. Not for any practical application but just because.
  I have a 40-90 on a Sharps action with an 18'twist barrel. It shoots good with bullets up to about 1.33" length. That gets me close to 400 grain depending on the bullet design. 
This little 40-50 with a 26" barrel length looking through the barrel appears to make two turns in its length. That would be about a 13" twist so should handle quite a long bullet. 
   I have quite a few 40 caliber molds that will run well over 400 grain so it should be fun trying. The duplex load seems to be working and is in the velocity range of original BP loads so chamber pressures should be in the ball park too.
   Going to a much heavier bullet I expect quite a reduction in velocity staying with BP type pressures.

I tried two other types of brass today to form 40-50 ss.  One was RP 30-40 and one was Hornady 405 WCF.
To my surprise the RP 30-40 brass did not require neck turning to chamber a round with a .408" bullet seated.
The 405 brass will require neck turning because with the neck expanded with a .407" inside neck expander same one used in the RP 30-40 brass the Hornady 405 brass will not chamber. No bullet seated just neck expanded with a .407" expander.
The Hornady 405 brass also required a different shell holder as the rims were too large to fit the 30-40 shell holder. The 405 rims are thicker and larger in diameter.
I tried to chamber one formed from the 405 brass full length sized but without expanding the neck and it did chamber but just barely.  The rim recess in the barrel is maxed out for diameter and thickness but the breach block did close with a bump from the ball of my hand. Looks like RP 30-40 brass is the ticket for this rifle.