While researching another post, I read of a .40 cartridge that involved turning down the rim, and shortening a 45-70 case to .44 mag length, then necking the case down to .40 cal. Ruger chambered some prototypes of the old .44 carbine in this ".40 magnum" and claimed higher energy levels than a .30-30.
Don't know what they used for slugs, as the .40 as we know it was decades off. I guess .38-40 slugs. The engineers loved it, but Winchester would not load it, nor anybody else for that matter I guess.
I reckon the OP was all about the .40 S&W, but this hot .40 business I find really fascinating.