The .38/40 Winchester, also known as the .38 WCF, was introduced about 1884. It is more or less the .44/40 necked down to .40 caliber. Why is a .40 caliber cartridge named the .38? Nobody really knows!
It was developed for the Winchester 1873 rifle. Colt began chambering the Model P single action sometime in the 1880s for the .38/40.
Ballisticly, the modern .40 S&W is on par with the .38/40... a 180 grain bullet, at about 950 FPS from a handgun.
I load the .38/40 with black powder and cast lead bullets for an 1892 Winchester rifle.I get about 1195 FPS from a 24 inch barrel.
Winchester .38/40 ammunition is pretty expensive. Look at some of the cowboy action loads available from some of the smaller suppliers...check MidwayUSA, Midsouth, Graf & Sons, and others.

Check here:http://www.midwayusa.com/ebrowse.exe/browse?TabID=3&Categoryid=7544&categorystr ing=653***690***