You want something large and very visible. Large caliber and shiny metal finish like nickel or polished stainless steel.
Like Madison said, a large diameter muzzle, so you want a .44 or .45 caliber bore. The shiny metal will make the muzzle appear even larger as the dark hole of the muzzle contrasts with the shiny metal finish.
S&W N frames, Colt New Service, Colt Anaconda, Ruger Redhawk, ect., ect.