Take a good look at Hornady .405 WCF brass.
Long ago,long enough I forgot some details, I wanted a 40-70.The 30-40 Krag brass was close,but a little short.
When loading lead bullets BPCR,the soft lead bullet upsets.
If your cartridge case is short,the bullet will upset oversize as it clears the short case mouth,then get swaged or scraped back down as it hits the chamber mouth.Its not a good deal.Best if the case is chamber length.
In the SPG lube book,they show a good compromise in the 40-60 Maynard.You are not chambered in that,but SPG suggested it as ideal for forming from 30-40 or .303 brass.Still you are short for 40-70 SS.
Then Hornady came out with .405 WCF and it is nearly perfect for 40-70 SS brass.
There may have been some variation in interpreting 40-70 SS.Hard to say how C Sharps chambered them for rim thickness,bullet dia,etc.
I made up a fireforming device to fit a common old break action single shot 12 ga.I think I used an old junk milsurp bbl .I ran a 40-70 SS chamber reamer into it.Do drill it through with a .406 drill.You do not want the dia to choke down to .30 or 8mm.That might drastically increase pressure when the cream of wheat hit it.Length may have been about 6 in.I turned the OD to match a 12 ga shell,so it would chamber in the shotgun.
I used the Bullseye/Cream of Wheat method.Use Cream of wheat,not cornmeal.Try about 6 gr of Bullseye to start.Creep up the charge till the case blows straight.I think I'd not go much past 8 gr.
I had very high failure rate on once fired brass Very good results with virgin brass.
I made a hard maple dipper,that would throw about 6 gr.I took primed brass to the range,and a loading block,A funnel,Cream of Wheat,and a little toilet paper.I also took the same drill bit I used to drill the dipper.3/8 or 7/16 or so.
By hand I could slightly sighly increase the charge the dipper threw with the drill bit till I got results.
Put the charge of Bullseye in the primed case in the loading block.Fill it wth cream of wheat.Push a bit of toilet paper on top.No bullet of any kind.
Stik it in the chamber insert in the shotgun.Bang.Straight walled basic brass,with 40-70 taper.
I forget which taper reamer,but if you study an MSC tool catalogue...I don't recall if its taper pin reamer,B+S taper,0 or 00 Morse..but there is a series of taper reamer that matches the taper and size range of the40-70 SS.
I would not recommend barrel chambering with it as it has no pilot.It will work for the fireforming chamber.They are cheaper than a chamber reamer by a bunch.
I told you all that to answer your question,but,the brass will still be short.
I gave the setup to a buddy who made a 40-50 SS.It works great for him.
Seems like original 40-70 used a .406/407 bullet of modest..maybe 360? gr bullet.The silhouette shooters would more likely use 400ish gr bullets around .410/411 dia.I think the .405 Win/Hornady is .412,but check.
You will be figuring out neck tension,neck/chamber clearance,etc.that works.
