The .44-40 WCF (Winchester Center Fire) was developed for the Winchester 73 repeating rifle. The Colt Frontier or Frontier Six-Shooter was Colt's 1873 "Model P" type revolver manufactured in . 44-40 Winchester caliber instead of . 45 Colt (in which configuration it was called the Single Action Army) so that it was compatible with Winchester Model 1873 ammunition. Production began in 1877.
The X's bullet is called a Dum Dum. The practice started in Dum Dum India. The notch makes the bullet expand.