These guns are a whole ball of fun to shoot. As Tiriaq says, brass can be formed from 38 Super. Fresh factory ammunition is made by Fiocchi, if you can find a dealer. I made my first brass for this thing out of .223, but making it from Super is a LOT easier! Correct base diameter is .380" and there is .005" taper in the casing, so mouth diameter should be .375": it is NOT a straight case, but a-l-m-o-s-t.
Original performance of this thing was pretty impressive; muzzle energy was pushing 420 ft/lbs. I wouldn't know how to get that kind of performance with the powders we have currently, without running into some serious problems. But then, George Roth, the guy who designed the cartridge, was a bit of a nut for high performance.
I find the guns are very difficult to shoot in the modern "instinctive" style, but, if you use the sights on the thing, you can get some really good shooting. Recoil is not bad at all especially considering the power of the gun and that odd grip angle.
I would really get some Lubriplate onto the barrel locking lugs and into the locking recesses of the frame and the barrel, in order to prevent galling, if you have any intention of shooting anything other than total-wimp loads. And one other point: the 1911/1912/1913 Steyr pistol (same gun, depending on when yours was contracted) was very tolerant of variations in pressure and has a tendency to work well with anything that will fit in the chamber, almost.
Fun toy, DO have a ball with it.