Still don't have my rifle so cant yet find out on my own.  Prepping some brass made from 303 Brit and wondering if the necks will need to be thinned. 
The necks at the case length for the 40-50 at 1.85" are below where the shoulder was on the 303 case so getting pretty thick.
You folks shooting .408" to .410" diameter bullets are you having to turn case necks from cases made from 303 Britt or 30-40 Krag?

Thank you for that information Sir Vbull .  It is very helpful. I am thinking that since you are sizing to 38 caliber and I am sizing to 40 caliber that my case necks will be thinner than yours.
I am sizing cases using a set of RCBS 40-70 SS dies because that is what I have. This die set has an inside neck expander of .407" diameter and it is definitely expanding the case necks.
I used a set of case expanding mandrels to initially expand the cases as far down as they would reach then ran them through the 40-70 FL die.
All I can do now is wait for the rifle to get here then further the learning process.   

The 40-50SS seems like it would be ideal for .40 smokeless. I do not currently have a .40 and was thinking if one is in my future it should be a 40-50SS.

My apologies on my reply mentioning .40-70SS as I saw the mention of using .40-70SS dies and got confused. 
I use both .303 British or .30-40 Krag brass cut down for my two .40-50SS rifles, and never have had to thin case necks using a .409" bullet for my .408" groove barrel. I rough cut the cases at the shoulder and then run them through my Lyman M die with a .410" expander. Then final trim to length and clean up the cuts. My bullets finger seat easily and still chamber without thinning the necks inside or out.
My loads use 4198 on an original BP action Rolling Block. I have a 34" Green Mountain barrel, and I'm loading with 4198 and a 370 grain cast bullet RCBS CSA mold. My loads are kept to BP levels at around 1375 fps and do great out to 500 yds. Pretty good at 650 yds. even, but haven't tried these lighter bullets out further.