Hello,
  I was just wondering if anyone knows of a good reference book for
identifying markings on shell casings.  We are currently running a project
and found a .22 rimfire shell with the markings "U.S." stamped on the end.
Did the U.S. military or the United States Cartridge Company stamp their
cartridges?  Any information will be greatly appreciated.
 
                                        Thanks,
                                                Karen L. Mayo