Yes, swagers have been doing what you have done for many decades. Cartridge brass is quite acceptable for making jackets for certain bullets. One of the downsides is that the bullets usually are longer for the same weight in copper tube or commercial jacket bullets. Another is that the ogive will be incomplete unless you use a standard Point forming swage die rather than some reloading type seating die. ( an incomplete ogive is not necessarily a bad thing). A big upside is that often cartridge cases cost a bit less than commercial jackets... and in some cases you have to use a cartridge case as you can't even find, or even reduce, a commercial jacket in the diameter you need.
