Just to change gears and make things very easy to reload, if you are going with smokeless powder, why not the 44/40.  About as powerful as the 40/50SS, enough power for 100yds shots and its easy to buy off the shelf cases and bullets.  Its done great on white tail deer and black bear for 145 years.
Rich

My action is not a CPA but a Stevens 44 1/2 so the receiver is too narrow for the 40-65.  If I use the Ruger or Browning I can use any case I want.  As far as length, when I was looking to rebarrelled my 44 1/2 2 years ago I was really thinking of going with the Krag so I asked Gail if an original action would work.  She was not positive because the distance from the breech  to the hammer might make it hard to load.  Lots of options. Still nobody is saying what bore and twist.  Those paper patched bullets look awesome!  What is the lightest 40 caliber pp bullet readily available?  Tom

Bulseyetom wrote on Feb 9^th, 2017 at 8:37pm:

Go back and read my previous post for bore dimensions. Again, the standard 40cal is 0.400" x 0.408" today. The standard modern 40cal twist has been 16". However, in the past 10-15yr, many of us prefer something faster to allow a wider choice of bullets. Green Mountain offers a 14.5" and Shiloh is now offering a 14". However, with the lighter bullets in the 40-50SS 16" would be fine. If you go with the 40-70SS you will likely want to consider the 14.5". 

Sadly, if you want to shoot any BPCR the Ruger will not be a legal rifle (like the Borchardt - no exposed hammer). Assume the B78 would be good as a modern repro of an 1885.

As an aside, three experienced shooters I shoot with have moved back from the 40-70SS to the 40-65 after a couple of years of frustration. YMMV.

Arnie - that GG bullet in the center of your pic looks familiar.

that 1.87 .40-60 wcf sounds like it' made a good southern-michigan legal rifle cartridge for deer hunting just have to trim it back to 1.8' to be legal----assuming the case it straight sided.   It'd even work in one of the BIG Ruger Super-Redhawks with a custom barrel and cylinder i imagine.

The 38-56 is the 45-70 necked down to 38cal and is a BN case. I tried for 3yr to get a High Wall with a 14" twist barrel to shoot. It would not shoot anything heavier than a 340gr bullet, even when stoked with 59gr of 2F Swiss. Finally gave up and built a 40-65, which is more accurate than I am. Think it would work better with a 12" twist and a 360gr+ bullet for hunting, but just thinking out loud. Being a BN case, it is tough to do a light load with BP.

The 38-50 is a very accurate round. Rick Moritz shoots one and has won Raton a number of times and was 2nd last year in some quite tricky conditions. The RBGC BPTR 200/300/600yd mid-range match last Feb was won by a 38-50 with a score of 290. Due to its length, the 30-40 Krag is the the starting brass. Hope to have my gain-twist 38-50 Hepburn finished before the end of year.

40-50 BN - easy to form from 45-70.  Surprised that no one has mentioned this number...

The old W.R.A. Co. 40-60 WCF cartridge that I've got sitting in front of me mics .502" at the web, just a few thousandths larger than my Starline 40-65 brass, so it is out as a contender for Tom's original 44 1/2, as would the 40-50 BN.

Indeed, I would go for the 16 twist and std bore/groove
of 0.400/0.408-0.409" along with one of those 
40s, 
Probably in the Ruger No 1 if dedicated as a hunting rifle. I have one, rebarrelled to 40-72. Uses std available Hornady
405W brass

I have been looking to rebarrel a M1886 Win parts gun
to 40-65 for a hunting rifle, and the above is what I 
will do hands down for bore/groove and twist
beltfed/arnie