Hello, Boss 541:
There are Threads on our SSS Forum and possibly in the SORI Section that discuss converting 50-70 Sharps cases to 56-50.
The conversion was common until Ken Kowell designed the STARLINE case, primarily for the slightly different chamber in the Armisport-Chiappa reproduction Model 1865 Spencer.
The people I know that were then selling such converted cases had BATFE Manufactuter licenses.
Presently, two custom case makers, Rocky Mountain Cartridge (RMC) in Cody, WY and Roberson Cartridge in Amarillo, TX, can supply 56-50 cases from bar stock with the nominal rim diameter of .650" versus the STARLINE's .640". Buffalo Bullets in Idaho has sold cases made from 50-70, but I am not sure they do now. Call Buffalo and speak with Dave Gullo.
There is a possible legal issue involved in converting and SELLING such cases. Doing so might put the converter-SELLER into a "manufacturer" classification for BATFE licensing. A friend was (emphatically) told by the BATFE two years ago at a SHOT SHOW that "he better get a license because manufacturing any of the four components of a cartridge, case, primer, powder or bullet, and selling them fits the licensing regulations".
I am not saying that one cannot convert for their own use. Best research this alternative.
I know from other firearms situations that for product liability exposure, "converting" is "altering", which puts the converter at risk if the work product is sold.
If one considers doing the conversion, the main tooling is a Forster case trimmer and collet that accept the .650" rim, a pilot for trimming the mouth and a reamer that enlarges the shortened case wall to match sized bullet diameter. Finding a reamer and altering it to fit the Forster tool is the challenge. There are several ways to shorten the case in Threads here.
Am curious what problem you are trying to resolve via the converted cases, please? Thanks.
Very respectfully,
El Supremo/Kevin Tinny
PS: It has been brought to my attention by a friend and SSS member that my ALIAS might raise eyebrows.
Sorry if so. I do not care for ALIASES, but was required to use one here. With the prior knowledge and permission of our Moderator, I chose a call-sign bestowed on me by a dear friend in a prior life. Smiles.